2o2 CcELHEL-MLXniES 



(lig. ^41.0) arc specialized paiis of ihc civlomic epitliclium and llicir prod- 

 ucts are usually carried lo the exterior by the iiephridia (_more rarely liy 

 ^T> .■"-.,-. special ducts), so that here, as in wrleli rales, we can 



speak of a urogenital s\'stem. A closed Idood system 

 is now present, now absent. Little in general can 

 he said of the nervous s)'stem; details ^vill ])e gi\en 

 in connection with the sepiarate classes. 



Class I. Chsetognathi. 



These marine forms, a half to two inches long, ]ier- 

 fectlv transparent, li\e at the surface of the sea, preying 

 on other animals, and from their shapes and rajiitl motions 

 descrye the name Siigitia — arrow — given some forms. 

 The animals swim by means of horizontal tins, one sur- 

 rounding the tail and one or two pairs on the sides of the 

 trunk (tig. 42"). On either side of the mouth are strong 

 bristles used in seizing prey (Ch;i?tognath, bristle-jaw). 

 Internally the body is separated into head, trunk, and 

 tail, bv transverse septa which di\ide the calom into 

 corresponding chambers. Each segiucnt of the civlom 

 again is divided into right and left halves by a mesen- 

 tery ^Ug. 241), supporting the straight intestine, running 

 lengthwise through it and terminating at the anus at the 

 end of the trunk segiuent. 



The nervous system is entirely ectodermal. In the 

 head is a pair of fused cerebral ganglia (tig. 24,0. in the 

 trunk segment a large ventral ganglion, and these are 

 connected by long a^sophageal conuui.^sures. Of inter- 

 est, because characteristic of nematodes and luany an- 

 nelids, are the relations of the musculature, which 



-ho 



Ovd ■ 



'J^t 



Fig. 242. Fig. 24,;. 



Fir.. 242, -Sui;ill.i li,:\<ij'hr,i, \i.'nli-al \icw (after (1. ITcrlw ij;V d. anus; I't;, 

 ventral lJ;a^^lilln; 1/, inlcslinc; /'/, I'lii; lin. Icsirs; 111. in.iiuli; o:', <i\-,n-\-; ,'ei/, oviikut; sb, 

 seminal vesirle; .st, (vso|iha;2ral roiunii.smiro; if/, lail I'm; s/. ,siK-rm; -,(':', female 0|ieniii,L;. 



Fin. 24,:;. — Meail of .S,i:;ill,i hi f>uii,i,ila. .Ku-sal \iew latter O. llerlwii;), ,ui. 

 nerve lo au. eye; ^'. lirain; ,Ji. hrisiles; 111, nerxes to ro, olfaitorN' or,u:ans; ,vr. ivsophaLjeal 

 commissure. 



consists of longitudinal llbres alone. The bodv cavity is lined with eiii- 

 thcHum (_lig. 241), whieli, where it ainits against tlie aliinentarv tract, is 



