HEMICHORDATA 



CHAP. 



C.TTV. 



of the dorsal body-wall, a conspicuous external feature of the 

 species in which they are present. The most interesting 



peculiarity of the digestive 

 tract in this region is the 

 existence, in certain species, 

 of pores, possibly vestigial gill- 

 slits, leading from it to the 

 exterior. 



Notochord and Skeleton. 

 The structure compared by 

 Bateson with the Vertebrate 

 notochord is a hollow dorsal 

 outgrowth of the alimentary 

 canal of the collar-region (Fig. 

 7, ri). Near its origin it is 

 slender, but in the proboscis 

 it dilates into a comparatively 

 large organ, which in most 

 cases retains its cavity. Its 

 cells have a vacuolated appear- 

 ance, which recalls the fine 

 structure of the Vertebrate 

 notochord. In Schizocardium 

 and Glandiceps, the organ is 

 produced into a slender " ver- 

 miform process " (v\ which 



extends nearly to the tip of 



. J 

 the proboscis. 



* 



The main Support of the 

 ^v^V^^o ofolV i<, j-V,P " nro 



boscis - skeleton " (s), a A- 

 shaped organ whose median 

 part lies beneath the base of 

 the notochord, its diverging 

 legs extending backwards 

 along the outer side of the 

 alimentary canal of the collar. 

 The proboscis -skeleton, like 

 the branchial skeleton, is a special development of the structure- 

 less membrane which is found at the base of the layers of cells 



b'.c? rre ---a. 



FIG. 7. Schizocardium brasiliense ; longi- 

 tudinal, median section through the pro- 

 boscis, the collar, and the first part of the 

 trunk ; b, main blood-space of the pro- 

 boscis ; b.c 1 , b.c 2 , b.c 3 , first, second and 

 third body-cavities ; c.m, circular muscles 

 of proboscis ; e, epidermis ; l.m, longi- 

 tudinal muscles of proboscis ; m, mouth ; 

 n, notochord ; n.s, central nervous system, 

 continuous with the subepidermic nerve- 

 plexus (black) of the proboscis, and with 

 the dorsal nerve (d) ; p.c, pericardium ; 

 p.s, proboscis - stalk ; s, proboscis - skele- 

 ton ; r, vermiform process of notochord. 

 (After Spengel.) 



