ANNELIDA 



307 



r 



■Brands (Fig. 144 B, mes). Consequently the larva of Thalas- 



sema possesses the greatest similarity to the Trochophore 



i^Hof the other Annelida. The same applies to the larva 



{■^fof Bchiurus (Figs. 145 and 146), the structure and meta- 



''"' morphosis of vi^hich 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 



R 



III, 



I 



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 blastocoele, 



there appears in EcMurus 



under the ectoderm a fine 



membrane, which arose by 



the union of branched 



mesodermal cells and is 



characteristic for this 



larva. The mesodermal 



bands are developed in the 

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

 osterior 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 



^-on 



— m 



Fig. 144.—^ and B, gastrula stage and 

 Trochoplxore larva of Thalassema mellita (after 

 Cown). a, anus; d, intestine; m, mouth; 

 mes, mesodermal bands; ces, oosophagus; 

 sp, apical plate. 



