DEVELOPMENT OF BALANOGLOSSUS. 



397 



posterior ring of cilia ; the primitive gut forms five ccelomic 

 pouches ; a mouth and an anus are formed, but there seem 

 to be no fore gut nor hind gut invaginations. 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 

 ventral mouth opens into the curved gut, which is furnished with a 

 posterior terminal 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 Miiller ranked the Tornaria 

 larva, whose adult form was not then 

 known, beside the larvae of Echino- 

 derms. The ciliated bands of the Tor- 

 naria resemble those of Echinoderm 

 larvae, but this is only a superficial 

 characteristic. The anterior pouch, 

 which forms the cavity of the proboscis 

 and communicates with the exterior has 

 also been compared with the beginning 

 of the water vascular system in Echino- 

 derms, and it is true that in both several 

 independent coelome pouches grow out 

 from the primitive gut. But we might 

 with as much force compare the Tor- 

 naria to an Annelid trochosphere, and it may be that it would be most 

 profitable to compare certain features in the development of Balano- 

 glossus with that of the lancelet. 



Affinities with Vertebrates (especially emphasised by Mr. Bateson). 

 (i) " Notochord" A dorsal outgrowth from the anterior region 

 of the gut grows forward for a short distance into the pro- 

 boscis and becomes a solid supporting rod (Fig. 126, Nch.}. 

 It may be compared with the notochord of Vertebrates, 

 which also arises dorsally from the gut. But it lies below 

 the main dorsal blood vessel, is of very limited extent, 

 and may be merely an analogue of the notochord a 

 physiological necessity for the support of the elongated 

 proboscis. 



FIG. 127. Tornaria 

 larva, from the side. 

 (After SPENGEL.) 



M, mouth ; -, gut ; a, anus ; k, 

 heart ; /, pore entering proboscis 

 cavity ; c.r, chief ciliated ring ; 

 s.c.r, secondary ciliated ring. The 

 dark wavy line indicates the mar- 

 gin of the lobes of the larval body. 



