ANNELIDA 



317 



ciliation meanwhile has degenerated. The further develop- 

 ment of the larva is quite simple. The body, which up 

 to this time was broad in front and narrow behind (Fig. 

 150), changes its shape to such an extent that it becomes 

 broader behind than in front. The principal changes in 

 the shape of the larva are brought about by the develop- 

 ment of parapodia, which takes place from in front back- 

 wards, as in the PolychiBta. Like the setge in Polychceta, the 

 hooks in Myzostoma are said to arise in ectodermal sacs. 

 A segmental differentiation of the compact mesodermal mass 

 lying between the integument and the intestine, a differen- 

 tiation which proceeds from in fi-ont backwai'ds, might be 

 compared with the segmentation of the mesodermal bands in 

 the Annelida. A large part of these mesoderm cells become 

 connected with the parapodia as musculature. Others are 

 applied to the mid- and fore-guts. The latter effect the 

 formation of the evertible proboscis. Up to this time the 

 intestinal canal has retained its simple character ; but by 

 the time the development of the parapodia is completed 

 evaginations are seen in it, and in this way its bianched 

 character takes its origin. As regards the formation of the 

 nervous system, the apical plate, which is to be looked upon 

 as a larval central organ, is said by Beard to degenerate ; 

 but since this author did not recognize the presence of a 

 supra- oesophageal ganglion and oesophageal ring, which 

 nevertheless are present, it is quite possible that the former 

 arises from the apical plate, and that, as in other Annelids, 

 a union with the ventral cord, which arises as an ectodermal 

 thickening, also occurs in the development. The ventral 

 cord, which exhibits, according to Nansen and v. Wagner, 

 the usual form of a chain of ganglia with transverse com- 

 missures, has thus a segmental arrangement. 



The statements concerning the origin of the mesodermal 

 structures are less certain. A true body cavity is not present, 

 but its place is occupied by parenchymatous tissue, which is 

 traversed by muscle fibres, and yet the authors (Nansen, 

 Beard) speak of an epithelium of the body cavity, from which 

 the sexual products atise. It appears, then, as if the hollow 

 spaces which contain the sexual pi'oducts constitute remnants 



