Habits of Polyclad Turbellaria.—Polyclads are exclusively marine, and for the most part littoral, animals. Moreover, there is no evidence of their occurrence in those inland seas where certain marine animals (including one or two species of otherwise characteristically marine Rhabdocoelida, p. [46]) have persisted under changed conditions. From half-tide mark down to 50 fathoms, some Polyclads probably occur on all coasts, but as to their relative abundance in different seas we have very little accurate information. The southern seas of Europe possess more individuals and species than the northern, and probably the maximum development of the group takes place on the coasts and coral islands of the tropics.[[27]] No Polyclads have been taken below 60 fathoms; but their delicacy and inconspicuousness render this negative evidence of little value. Six truly pelagic forms, however, are known,[[28]] and these are interesting on account of their wide distribution (three occurring in the Atlantic, Pacific, and Indian oceans), and also from the distinct modifications they have undergone in relation to their pelagic existence.
Whatever may be the interpretations of the fact, Polyclads are notoriously difficult to detect, and this fact doubtless explains the scanty references to them by the older naturalists who collected even in tropical seas. Lang, who worked seven years at Naples, added to the Mediterranean fauna as many Polyclads as were previously known for all Europe, in spite of the assiduous labours of his predecessors, Delle Chiaje and Quatrefages. Again Hallez, collecting at Wimereux at low-water, obtained some twenty specimens of Leptoplana tremellaris in an hour, while some other collectors working by his side could only find two or three. Yet, even making allowance for the difficulty of finding Polyclads, few of them appear to be abundant.
Leptoplana tremellaris is frequently associated with colonies of Botryllus, and if separated soon perishes, whereas the free-living individuals are distinctly hardy (Hallez). A closely allied but possibly distinct form lives upon the surface of the Polyzoon Schizoporella, on the French side of the Channel, and cannot long endure separation from its natural habitat, to which it is adaptively coloured. A striking case of protective mimicry is exhibited by Cycloporus papillosus, on the British coasts. This species, eminently variable in colour and in the presence or absence of dorsal papillae, is usually a quarter of an inch in length and of a firm consistency. Fixed by its sucker to Polyclinid and other Ascidians, Cycloporus appears part and parcel of the substratum, an interesting parallel to Lamellaria perspicua,[[29]] though we are not justified in calling the Polyclad parasitic. Indeed, though a few cases of association between Polyclads and large Gasteropods, Holothurians, and Echinids are known,[[30]] there is only one case, that of Planocera inquilina,[[31]] in the branchial chamber of the Gasteropod Sycotypus canaliculatus, which would seem to bear the interpretation of parasitism. The jet-black Pseudoceros velutinus and the orange Yungia aurantiaca of the Mediterranean, are large conspicuous forms with no attempt at concealment, but their taste, which is not known, may protect them. Other habits, curiously analogous with devices employed by Nudibranch Mollusca (compare Thysanozoon brocchii with Aeolis papillosa), emphasise the conclusion that the struggle for existence in the littoral zone has adapted almost each Polyclad to its particular habitat.
As regards the vertical distribution of this group on the British coasts, Leptoplana tremellaris has an extensive range, and appears to come from deeper to shallower water to breed.[[32]] In the upper part of the Laminarian zone, Cycloporus papillosus, and, among brown weeds, Stylochoplana maculata are found. At and below lowest water-mark Prostheceraeus vittatus, P. argus, and Eurylepta cornuta occur. Stylostomum variabile and Oligocladus sanguinolentus, though occasionally found between tide-marks, especially in the Channel Islands, are characteristic, along with Leptoplana droebachensis and L. fallax, of dredge material from 10 to 20 fathoms.
Locomotion.—Locomotion is generally performed by Polyclads at night when in search of food, and two methods, creeping and swimming, are usually employed—creeping by the cilia, aided possibly, as in the case of some Gasteropod Mollusca, by the longitudinal muscles of the ventral surface; and swimming, by undulations of the expanded margins of the body. In the former case the cilia work in a glandular secretion which bathes the body, and enables them to effect their purpose equally well on different substrata. The anterior region is generally lifted up, exploring the surroundings by the aid of the tentacles, which are here usually present. The rest of the body is closely appressed to the ground.
Swimming is particularly well performed by the Pseudoceridae, certain species of Prostheceraeus, the large Planoceridae, some Stylochoplana, Discocelis, and Leptoplana, and in the same manner as in Leptoplana tremellaris (p. [9]). In Cryptocelis, Leptoplana alcinoi, and L. pallida, however, the whole body executes serpentine movements like an active leech (e.g. Nephelis); a cross section of the body would thus present the same appearance during the whole movement. Many Polyclads, notably Anonymus (Lang), if irritated, spread out in all directions, becoming exceeding thin and transparent.
Fig. 9.—Discocelis lichenoides Mert. (after Mertens), creeping on the inner side of a glass vessel by means of the lobes of the extended and exceedingly mobile pharynx (ph). These lobes also serve to enclose Crustacea (a), and one lobe may then be withdrawn independently of the rest, back into the body (b). The brain (br) and shell-gland (sg) are shown by transparency.
Discocelis lichenoides, Planocera graffii, and Anonymus virilis have peculiar modes of progression. The first, according to Mertens, will climb up the sides of a vessel by means of the expanded lobes of the pharynx (Fig. 9, ph), a habit of considerable interest, since we know that certain Ctenophores—Lampetia, for instance—progress when not swimming on the expanded lobes of their "stomach."[[33]] Planocera and Anonymus creep by extending parts of the anterior margin and dragging the rest of the body behind. In consequence, the brain and dorsal tentacles may come to lie actually behind the middle of the body, and thus no definite anterior end or "head" advances first. Along with this curious habit it may be noticed (Lang) that the radial symmetry of the body is well marked; but even without accepting this author's suggestion of the concurrent development of a "head" with locomotion in a definite direction, the facts, whether these two forms are primitive or not, are highly interesting.
Food.—Though we are probably right in calling Polyclads a carnivorous group, the food of very few forms has been ascertained. Those which possess a large frilled pharynx (most Acotylea) probably enclose and digest large, and, it may be, powerful prey, as appears to be the case in Leptoplana tremellaris. Cryptocelis alba has been seen by Lang with the pharynx so distended, owing to a large Drepanophorus (Nemertine) which it contained, as to resemble a yolk-sac projecting from the under surface of an embryo. The Cotylea such as Thysanozoon, with a bell- or trumpet-shaped pharynx, are fond of fixing this to the side of the aquarium, but whether they thus obtain minute organisms is not clear. Prosthiostomum shoots out its long pharynx with great vehemence (Fig. 8, F) and snaps up small Annelids by its aid (Lang). Those Polyclads which, as Cycloporus and others, are definitely associated with other organisms are not certainly known to feed upon the latter, though "Planaria velellae" has been seen by Lesson[[34]] devouring the fleshy parts of its host. The salivary glands which open on the lips and the inner surface of the pharynx powerfully disintegrate the flesh of the prey. Digestion takes place in the main-gut, and the circulation of the food is accomplished by the sphinctral musculature of the intestinal branches (conf. Leptoplana, p. 13).