98 



BALANOGLOSSIDA. 



P' 



FIG. 74. Tornaria larva (after Metschnikoff), a from the side ; 

 b from the dorsal surface ; A anus ; C pericardium ; P, P' 

 coelomic sacs ; mouth ; S apical plate ; W rudiment of the 

 proboscis coelom (so-called water- vascular sac). 



eventually reaches the ventral wall. The hinder part of the proboscis 

 coelom thus becomes divided into two parts by a vertical septum. Some 

 of the cells of this septum give rise to the pericardium, which when first 

 formed is solid, and later on the notochord grows into it. 



The tornaria 

 larva. In most 

 species which 

 have eggs of the 

 smaller size, the 

 development is 

 indirect and a 

 transparent pela- 

 gic larva known 

 as the tornaria is 

 developed. This 

 larva was first 

 described by J. 

 Miiller, who took 

 it for an Echinoderm larva. Its real nature was determined 

 by Metschnikoff. The early stages of this development are 

 not known, but the later stages by which the larva passes 

 into the adult have been more or less worked out. In the 

 youngest stage known, the tornaria 

 has a somewhat ovoid form with a 

 ventral mouth, a terminal anus, and an 

 alimentary canal divided into three re- 

 gions, viz., oesophagus, stomach and 

 intestine (Figs. 74, 75). There are two 

 ciliated bands, one of which is preoral 

 and encircles the preoral lobe, and the 

 other postoral but longitudinal in direc- 

 tion. These two bands touch one another 

 dorsally and anteriorly, the ectoderm at 

 the point of junction being thickened to 

 form the apical plate (S). The apical 

 plate possesses nerve fibres and ganglion 

 cells and soon acquires a tuft of immobile cilia, 

 a pair of pigment ed eye spots. 



Internally there is a spacious blastocoel, in the front part of 

 which is a vesicle called the water-sac (Fig. 74). This is the 



FIG. 75. Early stage of it 

 tornaria larva in longitudinal 

 section (from Balfour after 

 Goette). m mouth ; an 

 anus ; w rudiment of pro- 

 boscis coelom (so-called 

 water-vascular vesicle). 



It also contains 



