BALANOGLOSSUS 



491 



with a ventral nerve. There is also a nervous plexus 

 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. 280, g].), and 

 connected with the 

 exterior by many gill- 

 slits, while the latter is 

 nutritive (Fig. 280,^.), 

 and conveys the food 

 particles onwards. Be- 

 hind 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, 

 hke 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. 



Fig. 278. — Male of Balanoglossus (Do- 

 lichoglossus) kowalevskii. — After Bate- 

 son. 



Note anterior proboscis. Mo., Mouth ; op., 

 slight operculum behind the collar ; then the 

 region with gill-slits ; is., testes ; a., anus. 



