290 



EMBRYOLOGY 



efinning of the body cavity — is formed in eacli segment of the 

 body of the worm. The cavity enlarges while the walls of 

 the primitive segments are more and more distended and 

 apply themselves to the body-wall and to the wall of the 

 intestine as the somatic and splanchnic layers respectively 

 (Fig. 133 C). But of course every two of the segments abut 

 on each other with their anterior and posterior walls, and 

 thus arise the septa (dissepiments), which separate the 

 different segments of the body. Since each segment of the 

 body requires for its formation a primitive segment on the 

 right side and one on the left, there are formed a dorsal and a 

 ventral mesentery (Fig. 183 (7). These mesenteries dis- 



til 



Fig. 133. — A to C, transverse sections of Polygordius larvae (after HiTSCHBK). 

 A, optical cross-section of the body of an unsegmented larva, immediately in 

 front of the anus, showing the two primitive mesoderm cells (mes) ; B, C, two 

 cross-sections of an older larva, the former from the posterior, the latter from the 

 anterior, part of the body ; ect, ectoderm ; ent, entoderm ; mes, mesoderm ; u, 

 fundament of the nervous system ; so, somatic, sp, splanchnic layer of the meso- 

 derm. 



appear in most of the Chaetopods (just as the septa also are 

 frequently perforated), but they persist in some of the lowest 

 Annelids, such as Polygordius among the Archiannelida and 

 Saccocirrus among the Chsetopoda. 



The body musculature arises from the somatic layer of 

 the primitive segments, the ventral longitudinal muscles 

 being the first to be formed. By the arrangement of the 



