ANNELIDA 



307 



bands (Fig. 144 7?, mes). Consequently the larva of Thalas- 

 sema possesses the greatest similarity to the Trochophore 

 of the other Annelida. The same applies to the larva 

 of EcMums (Figs. 145 and 146), the structure and meta- 

 morphosis of which were thoroughly studied by Hatschek, 

 who established the presence of a head kidney. This paired 

 organ consists at first of a simple canal, which opens to the 

 exterior on the ventral 

 side at the anterior end 

 of the mesodermal bands. 

 Later there is added to 

 this primary head kidney 

 a secondary branch, which 

 is much ramified (Fig. 

 145) . Altogether the larva 

 undergoes a number of 

 changes, until it arrives at 

 the height of its develop- 

 ment, and the larval organs 

 begin to degenerate. This 

 is true of some other meso- 

 dermal structures as well 

 as of the head kidney. In 

 addition to the muscles 

 characteristic of Annelid 

 larvae, which extend 

 through the blastoccele, 

 there appears in Echiurus 

 under the ectoderm a fine 

 membrane, whicii arose by 

 the union of branched 

 mesodermal cells and is 

 characteristic for this 

 larva. The mesodermal 

 bands are developed in the 

 manner typical for the Annelida. The pole cells lie at their 

 posterior ends, whereas the differentiation begins at the 

 anterior end. It is here that they are first many-layered, 

 and that they separate into the primitive segments. The 



oes 



'-- a. 



Fig. 144. — A and B, gastrula stagre and 

 Trocliophore larva of Thalassemamdlita (after 

 Conn), a, anus; d, intestine; m, mouth; 

 mes, mesodermal bands ; as, crsophagus ; 

 sp, apical plate. 



