BALANOGLOSSUS. 161 
blood-vessel. (4) Alongside of the stone-canal and opening into the 
madreporite is a long but small cavity called the axzal sinus. Part of 
its wall appears to form the so-called ovoed gland. 
The walls of the ccelom form muscles and the paired 
gonads which are situated inter-radially, opening dorsally 
to the exterior by fine pores. Scattered over the aboral 
surface are pores through which the ccelomic wall protrudes, 
as small vesicles or dranchie. The blood-vascular system 
of the starfish is represented, as in Badanoglossus, by sinuses 
in the mesenteries and possibly by a central heart. 
The radial nerves are connected with a nerve-ring round 
the mouth. Throughout its course the nervous system is 
an integral part of the ectoderm. An aboral nervous 
system has also been described. 
The eggs are fertilised in the water ; segmentation is total and equal, 
producing a blastula and gastrula larva. The gastrula is further dif- 
ferentiated into a free-swimming pelagic Azpénnaria. 
Development. This larva has a pre-oral and a post-oral ciliated band 
coiling over the surface of the body, and is serfectly 
plano-symmetric. Its mesoderm is early segmented into five principal 
parts, one pre-oral and two post-oral pairs. The adult grows like a 
large wart on the left side of the larva. ; 
This development is important, for it shows that the apparently axo- 
symmetric Echinodermata are descended from plano-symmetric forms, 
with an archimeric segmentation like that of the other Avchicelomata. 
II._BALANOGLOSSUS. 
PHYLUM ARCHICCELOMATA (p. 170). 
Sus-PHYLUM ARCHICHORDA (p. I7I). 
Balanoglossus is a long worm-like animal of a bur- 
rowing habit. It has the body divided into three segments. 
The anterior, or proboscis, lies in front of the 
mouth (or is pre-oral) and can be expanded or 
contracted at the will of the animal. The second 
segment or collar encircles the body immediately behind 
the mouth which is in the median ventral line. The third - 
part or ¢runk is long and forms the remainder of the body. 
At its extreme end opens the azus. In the constricted 
neck between proboscis and collar there open dorsally 
two small pores, the proboscis pores, which lead into the 
cavity of the proboscis. A similar pair of pores (the 
M. 12 
External 
Features. 
