TORNARIA. ECHINODERM AFFINITIES. 



99 



rudiment of the proboscis coelom. It lies on the hinder part of 

 the oesophagus and is connected to the apical plate by a muscu- 

 lar band, and opens on the dorsal surface by a short ciliated 

 tube, the rudiment of the proboscis canal and pore of the adult. 

 The pericardium (heart- vesicle), the origin of which is uncertain, 

 lies at first near the skin to the right of the proboscis pore. It 

 soon leaves the surface and becomes surrounded by the proboscis 

 coelom. By some authors it is regarded as the right proboscis 

 cavity. Later a transverse circumanal ciliated ring is formed, 

 and sometimes a 

 second less dis- 

 tinct ring behind 

 this. Moreover 

 the course of the 

 two first described 

 ciliated bands fre- 

 quently becomes 

 complicated and 

 folded, and in 

 Tornaria grena- 

 cheri they acquire 

 ciliated lobe-like 

 projections. 



In the later 

 development the 

 form of the adult 

 is gradually at- 

 tained ; the cili- 

 ated rings disap- 

 pear and the two 

 posterior pairs of coelomic sacs are developed in a manner 

 which has not been thoroughly ascertained. The collar nerve 

 cord, gill-slits etc., are formed as they are in the direct de- 

 velopment. 



The tornaria has by its ciliated bands and general form a 

 strong external resemblance to the bipinnaria larva of an echino- 

 derm, a resemblance which is increased by the presence of the 

 so-called water- vesicle with its dorsal pore ; and when first dis- 

 covered it was described as belonging to that group. A close 

 examination of its structure, though revealing points of difference 



FIG. 76. Two later stages in the development of Tornaria larva, 

 side view. A larva with one pair of gill apertures (after Metsch- 

 nikoff ) ; B larva, with four pairs of gill apertures (after A. Agas- 

 siz) ; Bo gill apertures ; C heart ; O mouth ; P coelomic sac ; 

 W rudiment of proboscis-coelom. 



