777. ANNELIDA. 



305 



Class III. Annelida. 



The segmentation of the body, which was shown in a slight de- 

 gree in the Chsetognathi, reaches its highest development in the 



Fio, 272. Diagram of annelid somites (orig.). am, acicular muscles; c, coelom; cm, 

 circular muscles ; ev, circular blood-vessels; d, dorsal blood-vessel; /, intestine ; 

 Im, longitudinal muscles; rn, mesentery; ?i, nerve cord; no, nephridium; ne, nv % 

 neuro- and notopodia, forming parapodium; s, septum; so, somatopleure ; sp, 

 splanchnopleure ; , typhlosole. 



Annelids, where it appears both in the outer ringing of the body 

 and in the arrangement of the most important systems of organs 

 metameric arrangement of excretory organs, nervous system, blood- 



FIG. 273. Trochophore (Loven's larva) of Polygordius. (From Hatschek.) A, anus; 

 dLM, dorsal muscles; ED, hind gut: /, stomach; J,, intestine; Mstr, mesodermal 

 band; ?i, nerves; Neph, protonephridia; O, mouth; Oe, oesophagus; oeLM, oesopha- 

 geal muscle; SP, apical plate; vLM, ventral muscle; vLN, lateral nerve; Wkr^ wkr, 

 pre- and post-oral zones of cilia; WS, apical cilia; wz, adoral cilia. 



vessels internal segmentation. To this is added an extraordinary 

 increase in number of body segments (somites, metameres), which 

 can far exceed a hundred. We can thus define the Annelids as 



