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Show 152 DR. II. LYSTER JAMESON ON THE [Mar. 4, the cuticle, a deeper longitudinal layer (l.m.), and, on the ventral surface, a less defined tract of transverse fibres inside the longitudinal muscular coat. The suckers are lodged in a slight involution of the cuticle (PI. X V . figs. 5, 7). Their relative and absolute sizes in surface view are determined by the degree of contraction of their constituent fibres. The ventral sucker is about one-fourth or one-third of the total breadth of the body. The mouth is situated in the middle of the anterior sucker, and generally appears triangular in sections (PI. X V . figs. 6, 7, m.). The funnel-shaped buccal tube opens behind, by a narrow orifice, into the spherical muscular pharynx (ph.). This is followed by the short straight oesophagus (ce.), which, passing upwards and backwards, bifurcates to form the sac-like digestive caeca (PI. X I V. fig. 2 ; PI. X V . figs. 5-7, dig.), which are dorsal to the other organs. In the resting worm these caeca are greatly distended with yellowish granular material, doubtless derived from the tissues of Mytilus. Even the oesophagus is often tightly crammed with food. The digestive system in this condition occupies the bulk of the body, anterior to the ventral sucker, but when empty is much smaller. The posterior end of the pharynx is provided with a group of salivary glands (PL X V . figs. 6, 7, s.gl.). The epithelium of the digestive system consists of very large flat polygonal cells with conspicuous nuclei (PL X V . fig. 7, int.ep.). The individual cells can sometimes be distinguished in pressure preparations. There is an ill-defined supra-pharyngeal nerve commissure (PL X V . fig. 7, n.c.) and a pair of lateral cords. The excretory system (PL X I V . fig. 2 ; PL X V . figs. 5-7, ex.) consists of two enormous tubular sacs, extending to the anterior end of the body and converging to form a pyriform median vesicle, which opens by a pore at the hinder end (PL X V . fig. 6, ex.p.). The excretory tubes are generally quite full of opaque spherical granules, presumably of excretory matter. When treated with hydrochloric acid they become transparent (PL X V . fig. 6). In living specimens a few flame-cells can be seen in short, lateral, and apparently unbranched tubules given off by the excretory sacs. But the distended condition of the latter makes it difficult to ascertain their precise relations. The female organs are not developed in the resting larva. The worm is protandrous, and the male genital organs reach a conspicuous size, even in the Sporocyst. The rudiments of the testes, vasa deferentia, and penis are very obvious in sections (PL X V . figs. 5, 7) and in stained preparations of the entire worm (PL X V . fig. 6). Being composed of young cells they stain deeply. In fresh specimens they are less obvious. The penis opens out at the genital pore (PL X V . fig. 7, g.p.), which is situated immediately in front of the anterior border of the ventral sucker. It is an elongated hollow pyriform body, lying in front of and dorsal to the sucker. The rudiment of the |
