ANNELIDA 



289 



The origin of the sensory organs lias already been touched 

 upon several times, as, for example, the formation of the 

 auditory vesicles in Eupomatus (comp. p. 267). The eyes of 

 the Alciopidoe are formed, according to Kleinenberg, as 

 invaginations of the ectoderm, which are constricted off and 

 unite with the brain, this union constituting the optic nerve. 

 The retina arises as the result of the differentiation of the 

 inner [deep] wall of the vesicle, whereas the outer wall 

 becomes very thin. Within, the lens and, by the activity of 

 certain gland-like cells, the vitreous body are secreted. 



Mesodermal Structures. 



Body Cavity ; Musculature ; Blood-vessels. — The differentia- 

 tion of the mesodermal bands, which results in the formation 

 of the segmental cavities, and thereby causes the segmenta- 

 tion of the body, takes place in a simple manner. The meso- 

 dermal bands have extended forward and in the anterior part 

 of the body become several rows wide and several layers 

 deep. Then a segmentation begins at their most anterior 

 end, individual parts becoming differentiated in groups, and 

 finally separated from one another by transverse boundaries 

 (Fig. 131, p. 285). These box-like, quadrangular cell-plates, 

 which succeed one another along the course of the meso- 

 dermal bands, and therefore lie side by side in two rows, 

 are the primitive segments, the influence of which on the 

 segmentation of the body we have already briefly men- 

 tioned in considering the development of Polygordius and 

 Lumhricus (pp. 268 and 284 — 286). We saw there also that 

 the development of the primitive segments takes place from 

 in front backwards. When the primitive segments are 

 already well formed in the anterior part of the body, the 

 mesodermal bands are still entirely undifferentiated in their 

 posterior portions, and new cell material continues to be 

 formed hei'C in the vicinity of the pinmitive mesoderm cells 

 (Fig. 131). A fissure soon makes its appearance in the 

 primitive segments, owing to the fact that the two or more 

 cell-layers out of which they are composed separate from each 

 other at the middle of each primitive segment (Fig. 133 B 

 and C). Thus the segmental cavity — that is to say, the be- 



K. H. e. U 



