272 



EMBRYOLOGY 



thorougli knowledge of the Annelid larvae, point out that 

 the characters cited have no great morphological value, for 

 larvae occur in the same family, and even in the same genus, 

 which belong to more than one of these types. The differ- 

 ences in shape are probably due to differences in the mode 

 of life. Variations in regard to the development of the 

 locomotor apparatus — i.e., in the distribution and stoutness 

 of the ciliation — would easily follow, if the larvaa of closely 

 related forms adopted different m.ode<6 of life, as is actually 

 the case. Terehella larvoe (Terehella conchilega) are known 

 which must be placed among the Nototrochm, while others 

 belonging to this genus entirely lack the ciliated rings {Tere- 

 hella Meckelii). The former are good swimmers, and lead a 

 pelagic existence ; the latter, on the contrary, never move far 

 away from the masses of eggs from which they hatch, and 

 may sometimes develop into young worms, even in the jelly 

 surrounding the eggs. 



The presence or absence of the preoral band of cilia may well be im- 

 portant in the interpretation of Annelid larvae, for (according to Klei- 

 nenberg) this alone possesses a ring-nerve, which is said to be lacking in 

 all other bands of cilia that make their appearance, with the exception of 

 the so-called post-oral band, which stands in close relation to the preoral. 

 Even where posterior ciliated bands appear without the existence of an 

 anterior one, as in the Mesotroclut, the ring-nerve is said to be absent. 



But these conditions are as yet 

 too little studied to allow one 

 to base on them a distinction 

 between the larvffi. 



Out of the multitude of 

 variously formed Annelid 

 larvae, only a few of the 

 particularly characteristic 

 forms can be chosen. We 

 shall first consider the 

 unsegmented larvae. 



The simplest larvae of 

 the Annelids are un- 

 doubtedly those whose 

 body is covered with a 



Fio. 122. — A and B, so-called atrochal 

 Annelid larvae — A, of Lumhvicnnereis (?) 

 (after Clapabedb und Mbtschnikoff); B, 

 of Sternaspis scutata (after Vejdovskt). 

 cu, cuticula ; d, intestine; ent, entoderm. 



