ANNELIDA 



311 



of cells, which surrounds the central yolk-mass like a sac. On the front 

 end of this fundament of the intestine a conical appendage becomes 

 noticeable, the first indication of the oesophagus, which subsequently 

 breaks through to the exterior in the region of the anterior ciliated band 

 (probably behind it). Lying between ectoderm and entoderm is the 

 mesoderm, which has split into a somatic and a splanchnic layer in most 

 parts of the body, whereas in the head region it has the form of a com- 

 pact mass of vesicular cells. Besides the mesodermal elements which 

 are transformed into musculature and peritoneum, there exist still 

 others, which lie in the body cavity. These are transformed into 

 structures like blood cells, which float in the body fluid. It is only at 

 this time that the formation of the spacious body cavity is accomplished. 

 The vessels arise from the peritoneal lining of the body cavity. 



al^ 



Fig. 148. — Stages of development of JBoneilia (after Spengki,, from Balfour's 

 Comparatiue Embi-yologyj. A and B, larvae with anterior and posterior bands of 

 cilia. C, yoang Bonellia. oX, intestinal canal ; an.v, anal vesicle ; m, mouth j «, 

 fundament of the ventral hook j ze, excretory organs. 



At the same time with the internal processes of development described, 

 an external change of form takes place. The ciliation of the body 

 disappears for the most part ; the anterior portion of the body grows 

 considerably in length, and its ventral side, which is still ciliated, be- 

 comes depressed, in this way acquiring a spoon-like form (Fig. 148 C), 

 and thereby realizing a stage like that in Ectdurus. Later, projections 

 are formed on the prostomium where the eye-spots are situated. By 

 their further growth is brought about the bifurcation of the prostomium 

 which characterizes the female of Bonellia. 



Of the internal changes, there is still to be considered the further 

 development of the intestine, whose central yolk-mass becomes absorbed. 

 The mouth-opening breaks through the base of the prostomium, while 



