14 



HEMICHORDATA 



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, 7i). 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 

 ■T'-C? and Glandiceps, the organ is 

 n.S. produced into a slender "ver- 

 miform process " (v), which 

 extends nearly to the tip of 

 the proboscis. 



The main support of the 



proboscis-stalk is the " pro- 



(s), a A- 



FiG. 7. — Schizocardium hrasiliense ; longi- 

 tudinal, median section through the pro- 

 boscis, the collar, and the iirst part of the 

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

 boscis ; S.c\ 6.c^, h.c^, first, second and boscis - skeleton " 

 third body-cavities ; cm, circular muscles i, i i i . 



of proboscis ; «, epidermis ; l.m, longi- Shaped Organ whose median 



tudinal muscles of proljoscis ; ?«, mouth ; 

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

 continuous with the subepidermic nerve- 

 plexus (blacli) of tl]e proboscis, and witli leers 

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

 i:>.s, proboscis - stalk ; s, proboscis -skele- 

 ton ; t', vermiform process of notochord. 

 (After Spengel.) 



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 



part lies beneath the base of 

 the notochord, its diverging 

 extending backwards 

 along the outer side of the 

 alimentary canal of the collar. 



