THE TURBELLARIA 19 



secretion into its lumen. As to the structure of the intestine, it 

 is chiefly interesting from the fact that some of the epithelial cells 

 can thrust out pseudopodia and directly take in food particles. 

 We have thus a starting-point for that syncytial condition of the 

 hypoblast which occurs in the Acoela. 



The excretory system or " water- vascular system" was first 

 identified by Ehrenberg ; while 0. Schmidt added greatly to our 



FIG. VI. Figs. 1 to 7 illustrate the Structure of the " Proboscis" of the Proboscidae. 



1. The anterior end of Pseudorhynchus bifidiis, v. Gr. a, the introversible non-ciliatt<l tip 

 of the body ; b, the isolated strands of longitudinal muscles which act as retractors of this 

 proboscis. 



2, 3, and 6 refer to Macrorhynchus (after v. Graff). , the non-ciliated epidermis of the 

 proboscis ; b, the ciliated epidermis of the body, represented in all three diagrams by the 

 vertically shaded region ; c, the muscular coat of the body wall, which at x splits into two 

 sheets, forming (d') a sub-epidermal or outer sheath of the proboscis, and (d) the inner sheath. 

 The former (d') can be withdrawn from the epidermis which appears to slide over it, as in 

 3 and 6. Between these two sheaths and inserted into them at each end are the intrinsic 

 muscles (c) of the proboscis ; /, the retractor muscles (which are omitted in Figs. 3 and (i). The 

 points marked x, y, z deserve attention. In 2, z marks the apex of the everted proboscis ; 

 x, the point at which the muscular sheaths (ddf) separate ; y a point about half-way along 

 the side of the proboscis. In 3, the apex z and the upper half of the proboscis have been 

 entirely withdrawn by the active contraction of the intrinsic muscles of the proboscis, so that 

 the point y now marks the lip of a cup. The outer sheath has been pulled away from the 

 epidermis between x and y. In 6, the proboscis is entirely retracted, the side as marked by 

 the point y having followed the apex, and lies half-way down the cup. This further process is 

 partly due to the retractor muscles. As the inrolling of the sides takes place, the outer 

 sheath (d') resumes its normal position against the epidermis, and the gap m is reduced. 



4 and 7. The proboscis of Schizorhynchus (after Hallez). 4. The anterior end of the worm, 

 the proboscis being at rest, a, the aperture of the proboscis sac on the ventral surface of 

 the body ; b, the proboscis, consisting of two halves, leaving a channel between them, into 

 which open the ducts of glands (c), the duct on the right side is supposed to be cut away ; 

 these glands are probably poisonous and represent the scattered, diffuse glands on the pro- 

 boscis of Macrorhynchus and others; d, retractor muscles. 7. The proboscis of Scltizorhynchus 

 everted (the surrounding parts are omitted), e, the muscular sheath ; /, intrinsic muscles. 



5. side view of the anterior end of Hyporhynchus (after v. Graff), a, the entrance to the 

 proboscis sac ; b, glandular part of proboscis ; c, intrinsic muscles surrounded by the sheath ; 

 d, mouth ; e, pharynx. 



8-13 illustrate the chief varieties of pharynx in the Turbellaria. a, mouth ; b, pharynx ; 

 &', prepharynx, or pharyngeal sac ; c, intrinsic muscles ; d, entrance to intestine. 8. Diagram- 

 matic section across the middle of a Rhabdocoel. a, mouth leading into a " pharynx simplex " 

 whose muscles (c) are not separated from the parenchyma ; d, the intestine, the dorsal wall of 

 which, as well as dorsal body wall, is represented in this, but is omitted from the subsequent 

 diagrams. 



11. A "pharynx bulbosus," in which the muscles of the pharynx are cut off from the 

 parenchyma by the sheath (e) ; the pharyngeal sac is better developed. This type of pharynx 

 is very frequent in Rhabdocoela, and is universal amongst the Trematoda. 



9 and 12. The "pharynx plicatus" typically developed in the Polyclads consists essen- 

 tially of a circular, horizontal fold (b) of the musoular wall of the pharyngeal sac. The muscles 

 spread outwards to be inserted in the body wall and act as retractors of the fold. On eversion 

 (12) the folded membrane spreads outwards and envelops more or less of the prey ; the pharynx 

 is thus turned inside out. 



10 and 13. The tubular pharynx, typically developed in Triclads, but occurring elsewhere. 

 10 at rest, 13 protruded. It is essentially an acrecbolic introvert, which can elongate and con- 

 tract by the action of its intrinsic muscles, and is withdrawn into the pharyngeal sac by the 

 retractors ; in these diagrams there is no attempt to represent the complex arrangement of 

 muscular fibres in the substance of the pharynx itself. 



knowledge of the general plan of the system. Typically, there are 

 in the Ehabdocoela two main canals, each with a pore posteriorly 

 (Fig. VII. 1), or they may unite to form a short common duct 

 before opening by a median pore ; and the fusion may go so far as 

 to give rise to a single median canal (Stenostomd). In Mesostoma 

 each lateral canal opens into the peripharyngeal sac by means of a 

 transverse canal, or (Prorhynchidae) the tw r o canals open anteriorly 

 by a median pore. The course of this set of tubules has recently 



