ANNELIDA ' 295 



I 



^^m cells from the somatic layer, which unite with the splanchnic layer 1 



Head Kidney and Segmental Organs. — In the larva of 

 Eupomatus the head kidney arises by the outgrowth of a 

 cell lying in front of each pole cell of the mesodermal bands. 

 Some other mesodermal cells take part in their formation, 

 for they supply the spheroidal cells which rest upon the 

 inner blind end of the head kidney and elongate into the 

 ligament-like threads of attachment of the organ (Fig. 118 B, 

 p. 265). In addition to the formation of the head kidney, 

 the few cells of which the mesodermal bands at first con- 

 sist are further employed for the formation of the larval 

 muscles. Only the two pole cells remain. These, by re- 

 peated division, supply the new mesodermal bands, which 

 Hatschek designates as secondary in contrast to those 

 primary ones which were early put to use. Later they reach 

 the great development which has already been described. 



According to Hatschek, the remaining segmental organs 

 originate from the head kidney, for a small ciliated canal 

 (in Polygordius), running in the somatic layer of the meso- 

 derm, branches oU from each head kidney at the junction of 

 its two arms. The nephridia are said to be given off from 

 this canal, one to each segment (Fig. 136). While the head 

 kidney degenerates, they reach their final development. 



Hatschek's description has met with little recognition, for it could not 

 be substantiated by subsequent investigators (Fraipont). However, the 

 discoveries recently made by E. Meyer on certain Terebellidae {Lanice, 

 Loimia) show the observations of Hatschek in a new light. In the two 

 Annelids mentioned the nephridia are united by means of a [longitudinal, 

 blindly ending, common] duct, which extends far backward. The dis- 

 charge takes place through as many [successive lateral] canals as there 

 are nephridia present, but these canals are connected with the nephridia 

 only indirectly, i.e. by means of the common duct. In the Capitellida 

 also, according to Eisig, connections between the different nephridia 

 exist in the form of ciliated canals. We do not intend to assert that 

 any great importance is to be ascribed to these conditions, for in the 

 first place their development is not known, and then the nephridial 

 system of the Terebellidae (and CapitellidsB) is shown to be essentially 

 modified. 



