392 



HEMICHORDA OR ENTEROPNEUSTA. 



equal ; a blastosphere or blastula results ; this is invaginated 

 in the normal fashion, and becomes a two-layered gastrula. 



The American species {B. kowalevskit) has a simpler 

 development than the others, for it is without a remarkable 

 larval form (Tornaria) which occurs in them. We shall take 

 the simpler case first, though it is probably less primitive. 



The blastopore or mouth of the gastrula narrows and 

 closes ; the external surface of the gastrula becomes ciliated ; 

 the endoderm lies as an independent closed sac within the 

 ectoderm. Meanwhile the em- 

 bryo 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 primi- 

 tive gut forms five ccelomic 

 pouches ; a mouth and an anus 

 are formed, but there seem to be 

 no fore-gut nor hind-gut invagina- 

 tions. The regions of the body 

 are defined at a very early stage. 



The Tornaria larva found in other 

 species is at first bell-shaped. A ven- 

 tral mouth opens into the curved gut, 

 which is furnished with a posterior ter- 

 minal anus. 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, becomes diffusely ciliated, acquires a 

 proboscis and two gill-slits, and thus approaches the form of the larva 

 first described. The further development is the same in both cases. 

 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 Miillei 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 larva?, but this is only a super- 

 ficial characteristic. The anterior pouch, which forms the cavity of the 



ys^ 



Fig. 172. — Tornaria larva, 

 from the side. — After Spen- 

 gel. 



M. 



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

 A., heart ; p., pore entering 

 proboscis cavity ; c.r., analring 

 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. 



