DEVELOPMENT. 



439 



endoderm lies as an independent closed sac within the ectoderm. 

 Meanwhile the embryo has become or is becoming free from the thin 

 egg envelope, and begins to move about at the bottom in shallow 

 water. It elongates and becomes more worm-like ; there is an anterior 

 tuft and a posterior ring of cilia ; the primitive gut forms five ccelomic 

 pouches ; a mouth and an anus are perforated ; there seem to be no 

 fore-gut nor hind-gut invaginations. Two gill-slits appear ; the regions 

 of the body are defined at a very early stage/- 

 In the indirect development, there is a Tornaria larva, at first bell- 

 shaped. A ventral mouth opens into the curved gut, which is furnished 

 with a posterior terminal anus. A "dorsal pore" leads into a thin- 

 walled sac which becomes the proboscis 

 cavity of the adult. There are external 

 bands of cilia, something like those of 

 an Echinoderm larva, and also an apical 

 sensory plate (like that of many Annelid 

 trochospheres), with two eye - spots. 

 The Tornaria is a pelagic form. During 

 its period of free pelagic life it gradually 

 loses its distinctive bands of cilia, be- 

 comes diffusely ciliated, acquires a pro- 

 boscis and two gill-slits, and thus ap- 

 proaches the form already described. 

 It is elongated in the post-oral region, 

 and becomes a creeping form. The 

 Tornaria must be regarded as the more 

 primitive larval form ; the temporary 

 absence of mouth and anus in the other 

 type is probably an adaptation acquired 

 after the pelagic habit was lost. 



Johannes Miiller ranked the Tornaria 

 larva, whose adult form was not then 

 known, beside the larvae of Echinoderms, 

 and the resemblance has been recently 

 emphasised by Willey. The ciliated 

 bands of the Tornaria resemble those 

 of Echinoderm larvse, but this is only 

 a superficial characteristic. The an- 

 terior pouch, which forms the cavity 

 of the proboscis and communicates 

 with the exterior, has also been com- 

 pared with the beginning of the water- vascular system in Echinoderms, 

 and it is true that in both several independent ccelom pouches 

 grow out from the primitive gut. The anterior body cavity in Balano- 

 glossus communicates with the exterior by a pore, which becomes the 

 proboscis-pore of the adult, and this has been compared with the 

 water-pore, or outlet of the water-vascular system of Echinoderms, 

 which similarly opens from an anterior enteroccel to the exterior. On 

 the other hand, the presence of an apical plate a structure almost 

 invariably absent in Echinoderms suggests an affinity with an AnneliH 

 trochosphere. 



FIG. 237. Tornaria larva, 

 from the side. After 

 Spengel. 



M., mouth; g:, gut; a., anus; 

 A., heart; ^., pore entering 

 proboscis cavity ; c.r., anal ring 

 of cilia ; s.c.r., secondary anal 

 ring. The dark wavy line in- 

 dicates the margin of the lobes 

 of the larval body with their 

 bands of cilia. Note also the 

 apical spot with cilia and sense 

 organ. 



