ANNELIDA 315 



and Harmer), the males, apart from the sexual characters 

 proper, are formed just like the females. 



IV. MYZOSTOMA. 



Myzostoma, the discoid parasite of the Crinoids, provided 

 with hook-bearing parapodia arranged in pairs, deposits its 

 eggs in large masses without bestowing on them any special 

 care. The eggs, which are enclosed in a structureless mem- 

 brane, are fertilized outside the parent, after the formation 

 of the two polar cells. The fertilized eggs sink to the 

 bottom. Their development was studied by Metschnikoff in 

 Mi/zostoma cirriferum, and afterwards somewhat more fully by 

 Beard in Myzostomum glabrum. The unequal cleavage leads 

 to the formation of an epibolic gastrula, of whose six inner 

 cells the two lying nearest to the blastopore are said to give 

 evidence by their darker appeai'ance of being mesodermal 

 cells. Tufts of- cilia soon afterwards make their appearance 

 on the ectoderm cells all ai'ound the ovate embryo, which 

 now breaks through the egg-shell. Its shape soon becomes 

 pyriforra. An ectodermal invagination, the fundament of the 

 mouth and fore-gut, makes its appearance in the region of 

 the blastopoi-e. It grows inward and unites with the 

 stomach, which in the meantime has been formed out of the 

 entoderm cells, which have increased greatly in number. 

 The anus arises at the posterior pointed end of the larva. It 

 is not easy to determine whether it also is formed by an 

 ectodermal invagination or merely by a fusion of ectoderm 

 and entoderm. In front of the anus there appears a papilla 

 [on the ventral side], which subsequently becomes quite large 

 and constitutes the end of the body, the anal opening thus 

 becoming displaced dorsad (Fig. 150). The subsequent 

 stages are chai'acterized by the fact that the cilia distributed 

 over the entire body become restricted to certain regions. 

 These are, first the anterior end of the preoral pai't of the 

 body, which constitutes the apical area and bears a tuft of 

 rigid cilia (Fig. 150), then a band of cilia lying immediately 

 behind the mouth and a second one in the region of the anus, 

 and finally a bundle of rigid cilia at the tip of the caudal 

 appendage (Fig. 150). At the same time with the changes in 



