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). [Illustration: FIG. 10.—Diagram of the musculature, causing peristaltic movements of the intestinal branches of Polyclads. (After Lang.)] A distinct vent or anus is always absent. After a meal the {25}faecal matter collects in the main-gut, and is discharged violently by the pharynx into the water. In a few species, however, the intestinal branches open to the exterior (Lang). _Yungia aurantiaca_, a large and abundant Neapolitan form, possesses such openings over the greater part of the dorsal surface; _Cycloporus papillosus_ has marginal pores; _Oligocladus sanguinolentus_ apparently possesses an opening at the posterior end of the main-gut; and _Thysanozoon brocchii_ frequently rends at this point, in consequence of the accumulation of food. RESPIRATION.—The oxygen of the atmosphere dissolved in the sea-water is, in default of a special circulatory fluid, brought to the tissues of Polyclads in two ways. The ciliated epidermis provides a constant change of the surrounding water, by which the superficial organs may obtain their supply; and the peristaltic movements of the digestive system, aided by the cilia of the endoderm cells, ensure a rough circulation of the sea-water, which enters along with the food, to the internal organs. The papillae of _Thysanozoon brocchii_, containing outgrowths of the intestinal branches, are possibly so much additional respiratory surface, although still larger forms (other Pseudoceridae) are devoid of such outgrowths.