86 WILLIAM BATESON. 



(1) The animal is ciliated and inhabits muddy sand. 



(2) The prseoral lobe is enormously developed. 



(3) The notochord arises at the anterior end of the hypo- 

 blast and grows forwards. 



(4) The origin of the central nervous system consists in the 

 delamination of a solid cord of epiblast in the dorsal middle 

 line of the middle third ; this, by invagination of its two ends, 

 s afterwards extended as a tube in both directions. 



Other collections of nerve-fibre are afterwards deposited in 

 various parts of the body, and finally a general network of 

 nerve-fibre occurs at the base of all the skin of the body, 

 especially in the line of the gill-slits. 



(5) The mouth originally faces ventralwards, but comes 

 afterwards to open forwards, being not a sucking but a dig- 

 ging mouth. 



(6) The gill-slits for a long time are only one pair, but 

 subsequently are repeated in pairs, increasing in number with 

 increase in the size of the body. 



(7) The mesoblast arises as one unpaired pouch, followed by 

 two pairs of pouches. 



(8) The blood system is entirely peculiar, consisting of an 

 anterior heart and a dorsal and ventral vessel, and in B. 

 minutus of two lateral vessels in the intestinal region. The 

 two former are united by a plexus of trunks, which are placed 

 under the skin and below the walls of the gut. 



(9) The generative organs are repeated through a large part 

 of the body ; in the branchial region more or less following the 

 repetition of the gill-slits. 



(10) Of the excretory system little can be affirmed. The 

 cells of the mesoblast appear to have a power of forming 

 concretions, probably excretory, in their substance, and 

 then throwing them into the body cavity. Here they 

 form small aggregations. A iai'ge gland (containing a 

 plexus of vessels), apparently performing their function, 

 exists in tbe proboscis cavity attached to the end of the 

 notochord. 



From the proboscis cavity opens an asymmetrical ciliated 



