BALANOGLOSSUS. i6i 



blood-vessel. (4) Alongside of the stone-canal and opening into the 

 madreporite is a long but small cavity called the axial si7itis. Part of 

 its wall appears to form the so-called ovoid gland. 



The walls of the coelom 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 coelomic wall protrudes, 

 as small vesicles or branchice. The blood-vascular system 

 of the starfish is represented, as in Balanoglossus^ 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 Bipinnaria. 

 Development. This larva has a pre-oral and a post-oral ciliated band 

 coiling over the surface of the body, and is perfectly 

 piano-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 piano-symmetric forms, 

 with an archimeric segmentation like that of the other Archiccelo?nata. 



II.— BALANOGLOSSUS. 



Phylum - - - Archiccelomata (p. 170). 

 Sub-Phylum - - Archichorda (p. 171). 



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 

 ea ures. QQ^iixdiCted 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 trunk is long and forms the remainder of the body. 

 At its extreme end opens the . anus. 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 



