BALANOGLOSSUS. 417 



often remains tubular, like a typical spinal cord. 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 larvae 

 of some species there are two eye spots. 



Alimentary system.- -The 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 constricted into a 

 dorsal and ventral region, of which the former is respiratory 

 (Fig. 206, g 1 .), and connected with the exterior by many 

 gill-slits, while the latter is nutritive (Fig. 206 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 hypo- 

 blastic structure from the dorsal wall of the gut in the 

 collar region. Beneath the notochord there is a more 

 superficial "proboscis skeleton." 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 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 primitive gut or 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 proboscis next the collar. A glandular 

 structure, which lies in the proboscis, may have excretory 

 significance, but it seems to be quite enclosed, (b} In 

 the collar region there are two small paired ccelomic 

 cavities, from which two funnels open to the exterior. 

 27 



