BALANOGLOSSUS. 165 



mesenteries — the dorsal and ventral vessels. l he dorsal 

 vessel runs forward to the "central sinus" which 

 lies just over the subneural gland — partly 



^ ^^* surrounded by the contractile pericardium. In 

 its course through the trunk the dorsal vessel receives 

 efferent branchials from the pharyngeal slits. The blood 

 appears to leave the central sinus and pass forwards to a 

 paired glomerulus. This is a glandular excretory organ 

 formed from the wall of the proboscis-coelom. From this 

 it finds its way into the ventral vessel round the pharynx. 

 The ventral vessel takes the blood back to the body. 



The course of the blood is thus as follows : — 



V)o^sdl vessel -^<<r 

 CenTrdl sinu3 



txcrefory organ 



^'^Venfrdl vessel 



Development. — The sexes are separate and the sexual products 

 are shed into the water. There are two types of development. In 

 one there is a larval development with a free-swimming pelagic larva 

 called Tornaria ; in the other there is a demersal larva with direct 

 development. The main facts in the direct development are as 

 follows : — 



1. Total equal segmentation to form blastula larva. 



2. Invagination to form a ciliated gastrula which escapes from 



egg-membrane. 



3. The hypoblast gives off five archenteric pouches to form the 



five segments of the mesoderm, one pre-oral and two pairs of 

 post-oral segments. The exterior of the elongated larva 

 becomes marked off into proboscis, collar and trunk. 



4. Mouth and anus are broken through, the latter at the same spot 



as the closed blastopore. 



5. The subneural gland grows forward from* the front end of the 



pharynx and pliaryngeal clefts appear. 

 The larva Tornaria is a transparent pelagic form with three complex 

 ciliated bands, a pre-oral, a post-oral, and a peri-anal. The last is 

 motor and the two former are mainly trophic. It has a complete 

 alimentary systeui fi"om the first and differs mainly from the demersal 

 larva in the large size of the hDemocoele, the small mesodermic segments, 

 the early formation of the pericardium and the presence of eye-spots. 

 The pharynx of To7'naria appears to have paired chordoid areas 

 (pleurochords) found in adult allies. 



