BALANOGLOSSUS. 165 
mesenteries — the dorsal and ventral vessels. ‘lhe 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-ccelom. 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 :— 
Pe vessel 
Central sinus a 
{ Gils System 
Blood Vas- 
cular, 
Excrefory organ 
“Saal 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 Yornarda; 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 pharyngeal clefts appear. 
The larva Zornaria 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 system from the first and differs mainly from the demersal 
larva in the large size of the heemoccele, the small mesodermic segments, 
the early formation of the pericardium and the presence of eye-spots. 
The pharynx of Yornaria appears to have paired chordoid areas 
(pleurochords) found in adult allies. 
