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naturally, as in the Vorticello and other Infusoria. Müller noticed it in P. ciliata, Dugès in Derostoma leucopta, Otto Fabricius in P. vulgaris. Draparnaud and Dugès saw it several times in P. tentaculata. I have frequently experimented myself in this way, and have seen these creatures reproduce lost halves, or segments; but on chopping them into eight or ten parts, I have always found they died, doubtless from some causes not favourable to reproduction. An individual divided in a longitudinal fissure at the head will often exhibit the phenomena of two heads. Planariæ propagate by mutual contact, as in other animals; they are androgynous; and no doubt the presence
of two individuals is necessary for the procreative act. Dugès has witnessed the copulation in P. torva, and recently I have noticed it in the same species. According to the researches of Dugès, "the male organ consists of two parts, one of which is free, smooth, semi-transparent, contractile, and always divided into two portions by a circular constriction; it is traversed by a central canal, susceptible of being dilated into a vesicle, and is open at its free extremity, which is turned backwards; the second division is thicker, more opaque, vesicular, adherent to the contiguous parenchyma, and receives two flexuous spermatic canals. The free portion of this organ is contained within a cylindrical muscular sheath, which is adherent to the circumference of the base of the intermittent organ, and serves to protrude it externally. This sheath communicates with the terminal sac of the female apparatus near its outlet by a projecting orifice. The oviduct opens into the posterior part of the terminal sac; it is a narrow tube which passes directly backwards, and dividing into two equal branches, again subdivides, and ramifies amongst the branches of the dendritic digestive organ. Besides the ovary, there are two accessory vesicles, communicating together by a narrow duct, and opening into the terminal generative sac.” The Planariæ lay round or ovoid eggs, with a horny covering, containing three or four young ones. These cases are of a reddish colour, and may be found in numbers deposited singly on the leaves of aquatic plants, especially within the stems of Sparganium. I believe the young ones issue from the capsules in eight or nine days after they have been laid ; the young ones are minute and drab-coloured, about a line long; they exactly resemble the parents, both in form and manner of life; they grow rapidly where the water is fresh and the food plentiful.*
* Other modes of propagation have been observed to take place in some of the Turbellariæ, as by internal budding, or the young are at first larvæ unlike their parents. Professor Agassiz once thought that certain infusoria, as Paramecium and Kolpoda, wore Planarian larvæ ; but the researches of Balbiani bave
Planariæ soon die when taken out of the water, a drop of spirits of wine or other alcohol, kills them instantaneously and renders their bodies hard ; vinegar softens but does not dissolve them.
The Planaria belongs to the order Turbellaria, whose characters are thus given by Dr. Johnston:“Worms individual, locomotive, very rarely tubicolous, monocious, or diæcious, with or without eyes, the surface usually coloured, and sometimes in elegant patterns, transparent or opaque.
Body soft, parenchymatous or cavernous, flat or sub-cylindrical, naked and lubricous, covered more or less with vibratile cilia, and sometimes with papillæ, often very contractile, and polymorphous, and sometimes breaking up voluntarily into pieces, head continuing "with the body, or rarely, imperfectly defined, either without tentacula, or with two frontal or dorsal ones prolonged from the surface. Mouth either terminal or ventral, and in the latter case situated in the anterior third, or near the centre, or towards the tail; often furnished with a prehensile proboscis. Intestine dendritically branched, or undivided, with or without an anus. Zoophagous, but some appear to feed, occasionally at least, on decaying plants. Oviparous, or viviparous, very rarely multiplying by spontaneous division.
no suckers or discs, and progression is made by gliding or by natation. They are never internal parasites, but tenant fresh and salt water, and are found sometimes in moist places. The skin is very rarely iridescent, and there is no phosphorescent species.”
As Dr. Johnston's book* is probably in the hands of only a few readers it may be well to give here his classification of the British genera. The order Turbellaria he divides into two sub-orders, viz. I. Planarica and II. Teretularia. The first order alone concerns us at present; it is thus defined :
“The body, parenchymatous, flat, or flattened, usually only a little longer than broad, acephalous, with or without eyes on the dorsum in front. Mouth, a simple pore, often the aperture to a prehensile proboscis. Anus, none. Genital pore, posterior to the oval.” This sub-order, Planarica, is divided into three families, with their respective genera, as follows: negatived that idea. Whether the fresh-water Dendrocoels or Rhabdocæls erer go through a larval condition I know not. I have hatched scores, and always noticed that the young exactly resembled their parents.
* I am well aware that Dr. Johnston's catalogue of British pon-parasitic worms is far from complete; nevertheless, as it is the only English text book on the subject, and the work of a very accomplished naturalist, I have preferred in this paper to retain it. It must be confessed that at present the subject of the Planarian worms and other Turbellariæ is involved in obscurity. Identification of specios is difficult. Young specimen hare, it is probable, been sometimes taken as distinct species.
* Intestine dendritically branched. Family I.-PLANOCERIDÆ. Flat: the mouth nearly in the centre of the ventral surface, and furnished with a short proboscis, crenated or lobed at the orifice. 1.-Leptoplana. Multocular, the eyes
clustered : tentacula.
2.-Eurylepta. Multocular, the eyes clustered : two frontal tentacula.
3.—Planocera. Multocular, or eyless: two dorsal tentacula.
Family II.-PLANARIADÆ. Flattish : the mouth inferior, sub-central, with a long cylindrical proboscis, plain at the orifice.
4.-Polycelis. Eyes, many, in a marginal series.
** Intestine an undivided tube.
Family III.—DALYELLIDE. Mouth terminal, or sub-central, eproboscidean.
† Eyes, two.
6.-Dalyellia. Mouth, terminal.
9.-Opistomum. Body, elongate; mouth terminal.
10.–Typhloplana. Body, linear-oblong : mouth ventral, sub-central.
11.-Convoluta. Body, involute.
In the PLANOCERIDE, the body is thin, flat, and laterally expanded; the eyes when present, are clustered. The oral aperture is usually closed, and becomes almost indistinguishable, but the position of it, and of the proboscis, is marked by an oblong spot, near the middle of the ventral surface. This is always paler than the dorsal, which is commonly beautifully coloured. The motion is slow. The food is soft, either the juices of invertebrate animals, or the parenchyma of decaying algæ. All are marine, and propagate, probably, by naked cilated ova, undergoing no metamorphosis. In decay the body is diffluent, and decomposition has far advanced before life is extinguished. The species Leptopluna tremellaris and Eurylepta vittata, with its sinuated margin, and dotted, ear-shaped tentacles, are probably familiar to many sea-side explorers, being not uncommon under stones, and within the tangled roots of laminaria, but our business is with the freshwater, and not the marine species.
The Planariadæ contain the species of true Planario; and, as I have already given my observations upon them, there is no necessity to define the family again. It consists of two genera, Polycelis and Planaria, the chief characteristic distinctions between them being that Polycelis has numerous oculiform spots of a black colour, bordering the anterior margin of the animal; these spots extend to about a third of the length of the animal; whilst Planaria has two eyes, nearly parallel, situated on the back of the head.
Of the genus Polycelis, Dr. Johnston enumerates three British species, viz: P. nigra, P. brunnea, and P. felina. The two first are considered by some naturalists to be merely varieties; I am rather inclined to regard them as specifically distinct. P. nigra is extremely common in ponds and ditches; it is evidently the “limace aquatique noire," portions of which little creature Trembley used sometimes to treat his favourite hydrae with. The body is depressed, even, and very smooth; of a black velvet colour. Of an oval form when at rest : linear-oblong when moving. The head is slightly sinuated with a central projection in front, and two marginal ones: this is seen only when the creature is in motion; the oval proboscis is long, white, and cylindrical, with a plain but dilatable orifice; length, when extended, about five lines, and one and a half broad.
P. brunnea does not differ, perceptibly, from P. nigra, except in colour, which is a smoky-brown, with a dark mesial line very distinct; it is as common as the black species. P. felinu which, like the last named kind, is regarded by Diesing only as a variety of P. nigra, is described as being linear-oblong in form, minutely tricuspidate in front, of a uniform dark brown, paler underneath, and eight lines long by one and a half broad. I do not know the species; it appears to be the Planarian viganensis of Dugès; and from the decidedly auricular form of the head, to judge from Dugès figure, ought perhaps, to be considered a distinct species. It inhabits stagnant waters, in which aquatic vegetables abound, and rarely is found in springs. Of the genus Planaria, Dr. Johnston enumerates four British fresh-water species, viz: P. lactea, P. torva, P. Arethusa, and P. Edinensis. The first named species is white or pinkish, and shows most clearly the dendritic cavity of the digestive system. When moving it is of an oblong form, the front truncate, a little auricled on each side; but it assumes various forms, and frequently sinuates its margins in elegant folds. The normal number of eyes is two, but there are occasionally four, in which case the anterior pair is very minute.
Dr. Johnston says it inhabits cold springs and lakes, is gregarious, and not common. I find it in the canal near my house, and in pools in the neighbourhood, tolerably abundant, and have never any difficulty in procuring specimens for examination. I do not consider it more gregarious than other species.
P. torva, This is a very quaint looking fellow; its two black eyes, which are of a crescent form, are partly surrounded by a white ring or halo, which gives the creature a squint-like look : suggesting, probably from its sinister appearance, a disposition conveyed by the epithet, “torva." It is described as being cinereous or black, on the dorsal, and greyish on the ventral surface. The front is obtuse, rounded on the angles, and projecting in the centre. It deposits a large oval capsule', and is six or seven lines long, by about two broad.
I have recently met with some very large velvety- black Planariæ, which resemble P. torva, in having a white halo round the eye spots; but I have reason for believing that they are individually of a distinct species. The black eyespecks in the surrounding uncoloured halo are not visible under a simple lens in the individuals I am speaking of; but the compound microscope of about twenty diameters reveals them. In P. torva, the eye-specks are very apparent under a simple lens: the colour is of a decided black when viewed by reflected light; in size it is almost equal to the large P. lactea, and it often crenulates its margins like it, but only very slightly. I do not find any other Pianarice like the ordinary P. torva in the water whence I obtain these large individuals, and I am inclined to regard them as, if not a distinct species, at any rate a well-marked variety. I have occasionally found a P. torva with four eyes, each pair with the characteristic white halo; the anterior ones in this case are small, as in P. lactea when possessed of four eyes. I have little doubt that this four-eyed variety of P. torva is the Tetracelis fontana of Diesing (“Systema Helminthum,” i., p. 191), who thus describes it : Corpus depressum ellipticum, antice truncatum, fuscum. Ocelli geminati nigri in macula alba, postici reniformes majores. Longit. 6"', latit. 1}"!.”
P. Arethusa is described as being truncate and auriculate in front, leaden or slate grey, paler underneath, having a black eye on a white spot on each side of the medial line in front. Length, six lines; breadth, one line. It is said to be common in pure springs and rivulets. I do not know this species.
P. Edinensis inhabits pure springs, and is rare. It is the smallest of the fresh-water Planario, being only tlıree lines