436 SUB-PHYLUM HEMICHORDA. 



beneath the epidermis. There are no special sense organs 

 in the adult. In the larvae of some species there are two 

 eye-spots. 



Alimentary system. The permanently open mouth is 

 on the ventral surface between the proboscis and the collar. 

 Sand seems to pass into it during the wriggling movements 

 of the animal, which are greatly aided by the turgidity of 

 the proboscis and collar. The pharynx is divided into a 

 dorsal and ventral region, of which the former is respiratory 

 (Fig. 235, g. 1 ), and connected with the exterior by many 

 gill-slits, while the latter is nutritive (Fig. 235, g.), and 

 conveys the food particles onwards. 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 diverti- 

 culum from the dorsal wall of the gut in the collar region. 

 Beneath the notochord there is a chitinous "proboscis 

 skeleton." The septa between the gill-slits are supported 

 by chitinous " forked primary " bars ; and each slit, at first 

 circular, is split into a V-shape by the growth downwards 

 of a double rod of chitin called a " tongue bar " ; the whole 

 is suggestive of Amphioxus. 



The body cavity. The body cavity consists of five 

 distinct parts, all of which are lined by mesoderm, and 

 arise as pouches from the archenteron. (a) There is first 

 the unpaired cavity of the proboscis, which communicates 

 with the exterior by a dorsal pore at the base of the pro- 

 boscis next the collar, (b) In the collar region there are 

 two small paired ccelomic cavities, from which two funnels 

 open to the exterior. Both these cavities and that of the 

 proboscis tend to be obliterated by growth of connective 

 tissue, (c) Two other cavities extend along the posterior 

 region of the body, to some extent separated by the dorsal 

 and ventral mesentery which moors the intestine. In these 

 there is a body cavity fluid with cells, 



