72 ART. 4. N. YATSU : 



among the fibres, are of a long spindle shape, compact, and rich in 

 chromatin which is in the form of fine granules. The peritoneal 

 lining around the muscle (sheath) cannot be seen readily as in the 

 adult, but the presence of it is certain. In all cases I have ob- 

 served that no cell layer intervenes between the muscle and the 

 outer layer of the body wall. The latter, the ectoblast cells, 

 (h. c.) undergo at the insertion of the muscle a peculiar change 

 and become muscle cells. They become elongated dorso-ventrally, 

 and their nuclei are displaced and attached to the cell wall. 

 These cells are named by Blochmann " Haftzellen " ('9 1 )- The 

 area covered by the " Haftzellen " is a little larger than the in- 

 sertion of the muscles. In a surface view of a larva each of 

 these cells is seen as a polygonal disc, its diameter greatly ex- 

 ceeding that of each muscle-fibre bundle. In the occlusor posterior 

 some thirty "Haftzellen" are present (PI. VIII., Fig. 128). 

 Physiologically considered the " Haftzellen " must play an im- 

 portant rôle. If the cells should remain unchanged as in the 

 original epithelium, the violent contraction of the neighboring 

 muscle would cause the detachment of the muscle from the shell. 

 The " Haftzellen," therefore, act as the tendon of mesoblastic 

 muscle-fibres, attaching themselves firmly to the entire muscle 

 on the one hand and to the shell on the other. 



Now we shall undertake the description of the shell-muscles 

 found in the larvse of the 5-6 p. c. stage. As the muscles are 

 very fine at this stage it is difficult to detect them in living 

 specimens. They are best studied in toto preparations (PI. VII., 

 Figs. 81, 82) and in sections {Cf. the Wood-cuts p. 73). At this 

 stage four pairs of muscles are found, viz. the occlusor anterior 

 (occ. ant.), the obliqmts internus, (obi. int.), the dorsalis (m. dr.) 

 and the ventralis (m. vt.). It should be noted that the last two 



