606 INVERTEBRATE MORPHOLOGY. 



either end, though free throughout the greater portion of its 

 length. It contains in young forms a central lumen, which 

 may be represented in adults by a series of separated cavities 

 and which results from its formation as an invagination of the 

 ectoderm. From this dorsal cord a plexus of nerve-fibres 

 extends all over the surface of the body, lying in the lower 

 layers of the ectoderm and being at certain regions specially 

 developed so as to form nervelike thickenings. One of these 

 surrounds the dorsal and lateral surfaces of the base of the 

 proboscis, being perforated by the proboscis-pore ; another 

 occurs at the posterior edge of the collar; while two others 

 occur in the trunk region, one in the dorsal (Fig. 275, B, nd) 

 and the other (nv) in the ventral mid-line, extending the entire 

 length of the trunk. No special optic, olfactory, or auditory 

 organs seem to be developed. 



The short canal opening by the proboscis-pore has been 

 regarded as excretory, but the assignment of such a function 

 to it seems questionable. A similar function has been as- 

 signed to two short tubes with folded ciliated walls which 

 communicate internally with the coelom of the collar, and 

 open to the exterior by the collar-pores, situated, one on each 

 side, on the edges of the atrial folds. More definite informa- 

 tion is required concerning these organs before they can 

 finally be accepted as excretory ; they evidently correspond 

 to the collar-pores of the Pterobranchia. 



All the known species of Balanoglossus are bisexual, the 

 reproductive organs, ovaries or testes, consisting of simple or 

 branched pouches situated in the trunk, beginning in the 

 braucial region and extending some distance backwards. 

 Each pouch opens to the exterior by a special duct, upon the 

 dorso-lateral portions of the body. 



Development of the Enteropneusta. Some species of Balano- 

 gJossus (B. Koivalewskii) develop directly without the inter- 

 vention of a larval stage in the life-history, but the majority 

 possess a characteristic free-swimming larva known as the 

 Tornaria (Fig. 277). It is a barrel-shaped organism, bulged 

 out slightly at either pole, and possessing a locomotor appa- 

 ratus in the form of somewhat complicated bands of cilia. 

 One of these surrounds the posterior portion of the body as 



