ox DEVELOPMENT ETC. OF PITOEOXTS. '3<U 



Mvoimd the poi-ipliery of the trunk (see fig. 12). Tii the Livva 

 of type C I have always found a comparatively thick layer of 

 circular muscles. In fig. öl b, which shows a portion of the 

 trunk wall containing the nephridial canal [nepx.), the muscles 

 are represented as a thin filn'ous layer [rir./j/.) intercepted 

 hetween the ectoderm and the peritoneal epithelium. The fioor 

 of tlie month just opposite the stomach diverticuhnn, is always 

 associated with a particularly well developed nmscular sheet. The 

 mesenchymatrms unicellular fibres which traverse the preoral cavity 

 are to be regarded as a kind of primitive muscles. The most 

 highly developed parts of the muscular system of the somatic, and 

 of the splanchnic, walls are to be found in the muscular sheaths 

 of the ventral pouch and of the dorsal wall of the stomach in 

 the advanced larv<r of all types. Each of them is formed of 

 a thick layer of enormously elongated muscular cells which stand 

 vertically to the ectoblastic, or the entoblastic, wall as the case may 

 be (figs. Ö8 c and ()8 e, in.sh.). The sheath of the stomach wall 

 is thickest along the mid-dorsal line of the stomach ; it is shown 

 in fio\ 58 c and fio-. <).3 e, the former figure beins; taken from a 

 cross section and the latter from a longitudinal section through 

 the dorso-antei'ior regiyn of tlie stomach. The muscular sheath 

 (or the external wall) of the ventral pouch is essentially similar 

 to that of the stomach. 



The Betracior 3ü(SirIes can be constantly detected in every 

 type of the larva as two slender threads on l)oth sides of the 

 œsophagus (figs. 12, 13, 14, l-j, ret.). They spring from the hind 

 lateral corners of the ganglion {(jL) and run divergently downwards 

 until they insert themselves in the collar walls between the first, 

 and the second, tentacles. In order to obtain a clear idea of the 

 position of these muscles, it is necessary to study them in sections ; 



