. Elements of Comparative Anatomy. the edge of tlie body; an elegant mesh-work may be formed by tlie connections of these with one another (Thysanozoon). Owing to the free commuuicatiou that these branches have with the central cavity, the chyme is distributed in the body, and so the enteric canal takes on the function of a vascular system. The land Plana- rians are remarkable, inasmuch as their enteric tube consists anteriorly of a median canal, while it is divided posteriorly. Nu- merous and regular transverse processes pass off from both divisions of it. The enteric tube is branched in many
. Elements of Comparative Anatomy. the edge of tlie body; an elegant mesh-work may be formed by tlie connections of these with one another (Thysanozoon). Owing to the free commuuicatiou that these branches have with the central cavity, the chyme is distributed in the body, and so the enteric canal takes on the function of a vascular system. The land Plana- rians are remarkable, inasmuch as their enteric tube consists anteriorly of a median canal, while it is divided posteriorly. Nu- merous and regular transverse processes pass off from both divisions of it. The enteric tube is branched in many of the Trematoda. The gut commences by a mouth, which is generally placed in the anteinor region of the body, and, as a rule, has the surroundiug parts meta- morphosed into a sucker (Fig. 69, s); this is followed by a muscular pharynx (6), from which the enteron proper is given off. This is, when most simple, a csecal sac (Aspidogaster, Gasterostomum), and corresponds to a low grade of develop- ment ; this is very common among the Trematoda at certain stages of their de- When more differentiated, the enteron divides into two branches, which pass backwards, and either give off greatly ramified branches into the body (Distoma hepaticum), or form simple caacal sacs (c) (Distoma flavescens, D. lanceolatum). The two branches may unite again and form an arrangement like that which obtains in some Planarise. It is clear, from the homogeneity of its struc- ture, as well as from its contents, that even in the Trematoda this branching of the gut is merely an eulargement of the tract in the body, and not the formation of heteronomous segments. The texture of the wall is in correspondence with the low stage of this form of enteron, for only the epithelial investment is indepen- dent, being bounded, exteriorly, by the tissue of the jDarenchyma of the body— connective tissue. Complete degeneration of the gut is clearly due to adaptations to definite modes of hfe, in which the food passes
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