DEVELOPMENT OF ANNELIDS. 183 



the apex, and furthermore, the formation of fore-gut and 

 hind-gut by anterior and posterior invaginations of ectoderm, 

 which communicate with the original gastrula cavity or 

 archenteron. The larval food-canal has thus a curve con- 

 cave ventrally. In the primitive body cavity (really the 

 segmentation cavity) there lie posteriorly a few mesoblast 

 cells which have a very varied future before them. 



Such a larva is called a trochosphere or trochophore. Its 

 chief characteristics are, (i) the prominent pre-oral region 

 with its apical ganglion and ring of cilia, (2) the three-fold 

 origin and concave curve of the gut, (3) the nature of the 

 larval body-cavity and the posterior primitive mesoblast 

 cells which lie in it, and (4) the fact that the future formation 

 of segments and growth into worm-like form takes place 

 in a posterior area in front of the anus. 



But we must notice a few complications of less constant 

 occurrence. The pre-oral ring of cilia is often accompanied 

 by a post-oral, and there may also be an ad-oral ring between 

 the other two, and also an apical tuft. The larva is thus 

 very efficiently adapted for free swimming. The apical 

 ganglion may be accompanied by a pigment (optic ?) spot and 

 other sensory structures. In the head there is often a pair 

 of larval excretory tubes which afterwards disappear. 



It is important to understand that this Trochosphere arises 

 by a gradual modification of the gastrula,- that most of it 

 corresponds to the future head, and the apical ganglion to 

 the brain, that the formation of segments takes place in 

 a posterior area of growth, between the post -oral ring of cilia 

 and the anus. This form of larva is very common, either 

 as such, or in some modified guise. 



Pedigree of Annelids. — According to Lang, the Chaetopods 

 are derived from a leech-like type, this from a Polyclade 

 Turbellarian, and this from a Ctenophore. According to 

 Sedgwick, the Annelids are derived from an Actinozoon-like 

 ancestor. But we cannot here discuss these possibilities, 

 nor the difficult questions concerned with the meaning of 

 segmentation or metamerism. 



