THE BODY CAVITY. 389 



and internal radial and longitudinal muscles. The fibres are 

 unstriped. 



Nervous system. — The dorsal nerve-cord is most 

 developed in the collar, but is continued along the whole 

 length. It arises as a solid cord of epiblast, which is 

 continued both forwards and backwards as a hollow 

 tube. The cavity is said to be comparable to that of 

 the spinal cord in Vertebrates. But the dorsal nerve- 

 cord is connected by a band round the collar with a 

 ventral nerve. There is also a nervous plexus beneath the 

 epidermis. There are no special sense organs, nor should 

 we expect them in an animal which spends most of its life 

 immersed in muddy sand. In the larva? of some species 

 there are two eye spots. 



Alimentary system. — The mouth is permanently open, 

 and is on the ventral surface between the proboscis and the 

 collar. Sand seems to pass into it during the wriggling 

 movements of the animal. The pharynx is constricted into 

 a dorsal and ventral region, of which the former is 

 respiratory (Fig. 170, g 1 .), and connected with the exterior 

 by many gill-slits, while the latter is nutritive (Fig. 170, g.), 

 and conveys the food-particles onwards. According to 

 Willey, there is no evidence that this groove is comparable 

 to a structure of similar appearance seen in Tunicates and 

 the lancelet, as used to be asserted. Behind the region 

 with gill-slits, the gut has a dorsal and a ventral ciliated 

 groove, and bears, throughout the anterior part of its course, 

 numerous glandular sacculations, which can be detected 

 through the skin. The anus is terminal. The animal eats 

 its way through the sand, and derives its food from the 

 nutritive particles and small organisms therein contained. 



Skeletal system. — The skeletal system is represented by 

 the " notochord," which lies in the proboscis, and arises, 

 like the notochord of indubitable Vertebrates, as a hypo- 

 blastic structure from the dorsal wall of the gut. Each gill- 

 slit is furnished with a " chitinous " skeleton, which gives 

 the slit a U-shape, on account of the growth downwards 

 of a "tongue bar"; the whole is suggestive of Amphioxm. 

 Beneath the branchial skeleton there lies a "chitinous" 

 rod, which divides into two in the collar. 



The body cavity. — The body cavity is somewhat complex, 



