HETERONEREIS STAGE OF NEREIS KOBIENSIS McINTOSH. 22Q 



and blood vessels, bv, Fig. I, indicating that loose strands of this 

 tissue, carrying vessels, still persist. 



In the posterior portion of the body, the so-called " modified " 

 region, similar conditions hold with respect to the septse and 

 intestine, but the body wall is very noticeably thicker, its muscle 

 layers being very well developed. This is easily seen by reference 

 to Fig. 2 which is a section through a posterior somite of the 

 annelid drawn in Fig. I. The longitudinal band of muscle fibers 

 is much more strongly developed, and the oblique fibers, espe- 

 cially those connected with the parapodia are vastly stronger than 

 those in the anterior somites. The difference in thickness of the 

 body wall is especially well seen in a comparison of Fig. 3, an 

 enlarged camera drawing of the body wall at X in Fig. I, with 

 Fig. 4, a similar drawing of X of Fig. 2. The dermal layer is 



l.m.--\ 



FIG. 3. Detail of ventral wall at FIG. 4. Detail of ventral wall taken 



A' in Fig. I. y( 280. li, longi- from the point 4 in Fig. 2. ( 280. 



tudinal muscle fibers seen in sec- cut, cuticle, 

 tion ; fp, epidermis. 



much thicker, and the longitudinal band is relatively enormously 

 developed. It will be understood, of course, that the thickness 

 of the muscle band is indicated by the entire diameter of the 

 figure. Owing to the irregular arrangement of the fibers no one 

 of them extended through the bundle in a straight enough fashion 

 to appear entire in a single section. It is evident further that the 

 arrangement in Fig. 4 is more nearly a normal one than in Fig. 

 3, so that the condition of the latter must be regarded as due to 

 degeneration. 



On either side Fig. 2 shows a section of a parapodium, that on 

 the right containing an ovum in its cavity. 



Similar conditions hold in the case of the male heteronereid. 

 The anterior " unmodified " portion of the body is more or less 



