DESCRIPTION OF BALANOGLOSSUS. 393 



through the soft sand. There are external circular and in- 

 ternal 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 v?hole 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 com- 

 parable to that of the spinal cord in Vertebrates. But the 

 dorsal nerve cord in the collar is connected by a band round 

 the pharynx 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 opens ventrally between the proboscis and 

 the collar, and is adapted for swallowing the sand moved 

 about by the wriggling proboscis and by the collar. The 

 pharynx is constricted into a dorsal and ventral region, of 

 which the former is respiratory (Fig. 125, ^i), and connected 

 with the exterior by many gill slits, while the latter is nutri- 

 tive (Fig. 125,^), and conveys the food particles onwards. 

 This ventral region may be compared with the " ventral 

 groove " in Tunicates, with the " hypobranchial " groove in 

 the lancelet, with a similar region in the lamprey, and even 

 with part of the thyroid gland in higher Vertebrates. 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 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 hypoblastic structure from the 



dorsal wall of the gut. Each gill-sht is furnished with a 



