2o8 Introduction to Animal Morphology. 



not continuous mesially above or below ; the circular 

 fibres are stronger below. Anteriorly there is a hollow 

 proboscis (sipho) for boring in the sand, with a round 

 opening at its free end 'and a triangular one below\ 

 This is separate from the digestive canal, and being 

 near to the'pharyngeal ganglion is probably a sense- 

 organ like the grooves of Nemerteans. The mouth is 

 large ; the intestine, for the most part fused with the 

 body wall, has lateral hepatic caeca containing a 

 greenish fluid, is often full of sand, and has a terminal 

 anus. 



The digestive canal in front bears a curious respi- 

 ratory organ, supported on a basket-work of homo- 

 geneous cuticular (chitinoid r) rods, which in arrange- 

 ment and genesis resemble the branchial skeleton of 

 Amphioxus. Between these gill arches are vertical 

 slits (like those of a Tunicate or of Amphioxus) for 

 the exit of the water which, entering by the mouth, 

 bathes the vascular, ciliated, branchial membrane 

 stretched on this framework. Each rod consists of 

 three vertical plates, joined by transverse bands, and 

 they are in contact at their outer end. These arches 

 only surround the dorsal half of this region of the in- 

 testine, and the incurving of the lower ends of the 

 arches constricts the cavity so as to make it, in vertical 

 section, somewhat resemble the figure 8. 



The intestinal wall within exhibits two ciliated 

 furrows, from which branching grooves pass in several 

 directions, forming a ciliated network. The ovaries 

 are red, the testes white, lying in the lateral parts of 

 the body, and shining through the skin. There are 

 usually four series of sex-glands. The embryos are 

 girdled by a single crown of cilia [Mecznikoff], and 



