xin 



PHYLUM CHORDATA 



(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 numberj 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 



Fm.'717. Development of Balanoglossus. A, stage of 

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

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

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

 (After Bateson.) 



cavities. At 



Jbrob cauf 



cards - 



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 

 differ in shape and 

 colour ; the ovaries 

 and testes are simple 

 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. 



cil.r 



FIG. 718. Tornaria. Dorsal view. an. anus ; card. .. cardiac 

 sac ; cil. r. post-oral ciliated band (membranellse) ; oil. r z . 

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

 cao. proboscis-cavity ; proh. po. proboscis-pore. (After 

 Spengel.) 



