the end, which is obtusely pointed. Anteriorly they are separated (see fig. 16) by a naked, 

 somewhat concave part, (extending a little downwards) here contiguous to the basis of the 

 remarkable oval shield (c) which, both by its enormous development, and peculiar function, 

 below described, forms one of the most peculiar features of the Rhabdopleura. 



The Bucal Shield. 



Between the bases of the tentacular arms, and from a somewhat ventral point, pro- 

 ceeds in an anterior direction, a large and very remarkable prominence (fig. 4. 5. &c. c) situ- 

 ated longitudinally, which has the form of an oblong thick disk or shield, one surface of which 

 (the dorsal surface) is in the middle grown together with the anterior end of the body, while 

 the ventral surface is free and bordered by a rather thicker raised ridge, distinct from the 

 adjacent parts. The form of this disk is, as above mentioned oval, or rather rounded pen- 

 tagonal (see fig. 18. c) nearly half as long again as wide; the width about equal to that of 

 the body at the beginning of the stomach. Its posterior border, which at once shews itself 

 (see fig. 5) separated by a deep constriction from the part of the body lying behind it, is in 

 the middle slightly incurved (fig. 18). The side borders are, a little in front of the middle, 

 strongly, almost angularly bent, and then converge strongly towards the anterior freely projec- 

 ting extremity, which is narrowed obtusely. The whole disk is everywhere, and especially 

 distinctly on the edges, thickly covered with small vibratory cilia. Allmann, who also mentions 

 this prominence, but without having gained a correct notion of its form and connexion with 

 the other parts, says of it that one might take it for a large x and peculiarly developed Epi- 

 stome, if its position on the ventral side of the mouth, and not, as in the fresh-water Polyzoa, 

 between the mouth and the anus, did not oppose such a supposition. Allman supposes thus 

 that the mouth in the Rhabdopleura, in analogy with the other Polyzoa, is terminal, and situ- 

 ated above, or on the dorsal side of this prominence, between it and the anus. Such is how- 

 ever not the case. The anterior extremity of the body above the prominence described is 

 completely closed without any trace of aperture (see fig. 16). On the other hand I have by 

 gently compressing the animal, been able distinctly to see (see fig. 15) that the bucal aper- 

 ture (q) is situated just on the ventral or haemal side behind that prominence, and seems to 

 have the form of a cross slit, which is bordered behind by an oval lobe (m.) furnished with 

 vibratory cilia like a sort of underlip. By increased pressure the bucal shield could be moved 

 more out from the base of the tentacular arms; and it appeared then everywhere very dis- 

 tinctly constricted from the rest of the body, and in the middle of the dorsal surface, fixed 

 to the body by a sort of short stalk, while the upper and the lower part were free. Further 

 it was observed that on each side of the bucal shield there extended, from the base of the 

 tentacular arms downwards, a strongly projecting nearly semilunar .border of thin skin (1) 

 ciliated on its edges, so that between this and the bucal shield there is formed, on each side, 

 a narrow half-tube or channel leading to the bucal aperture, and through which the nourish- 

 ment is probably conveyed to the mouth by the abundantly ciliated tentacles. Since, as above 



