xur 



PHYLUM CHORDATA 



FIQ. 717. Development of Balanoglossus. A, stage of 

 the formation of the first groove (gr.). B, stage in which 

 the second groove has appeared, and the first gill-slit has 

 become developed, co. collar ; g. si. gill-slit ; pr. proboscis. 

 (After Bateson.) 



(dors, n., vent, n.) which extend throughout the length of the body. 



These are merely thickenings, of a layer of nerve-fibres which 



extends over the entire 



body in the deeper 



part of the epidermis. 



Here and there are 



giant nerve-cells. The 



part of the dorsal 



strand which lies in 



the collar (collar-cord) 



is detached from the 



epidermis ; it contains 



a larger number of 



the giant nerve-cells 



than the rest ; in some 



species it encloses a 



canal, the neurocoele, 



opening in front and 



behind ; in others a 



closed canal ; in most 



a number of separate cavities. At the posterior extremity of 



the collar the dorsal 

 and ventral strands 

 are connected by a 

 ring-like thickening, 

 and there is a thick- 

 ening also round the 

 neck of the proboscis. 

 There are no organs 

 of special sense ; but 

 some cells of the epi- 

 dermis on certain 

 parts of the proboscis 

 and on the anterior 

 edge of the collar 

 seem to be of the 

 character of sensory 

 cells. 



Reproductive 

 Organs. The sexes 

 are separate , and often 



FIG. 718. Tornaria. Dorsal view. an. amis ; card. *. cardiac a ^ er m shape and 



sac ; cil. r. post-oral ciliated band (membranella?) ; cil. r 2 . Colour I the OVariCS 

 posterior ciliated ring ; eye, eye-spots on apical plate ; prob. -, , , i 



can. proboscis-cavity ; prob. po. proboscis-pore. (After ana tCStCS are Simple 



SlwnBeL) or branched saccular 



organs arranged in a double row along the branchial region of the trunk 

 and further back ; they open on the exterior by a series of pores. 



rob ecu/ 



ca roL s 



'rob po 



cil.r 



