W. M. Coe — Anatomy of Cerehratulus lacteiis. 483 



The Musculature. 



The muscles of the body-wall form a strong, muscular tube which 

 is almost entirely closed except where broken through by the alimen- 

 tary canal, proboscis-pore, and nephridial and reproductive openings. 

 The cutis rests directly upon this muscular tube, the wall of which 

 is very strong and thick in the oesophagal region but is much less 

 developed farther back. There are three principal layers of these 

 muscles ; an outer longitudinal (Plate XI, figs. 1, 3, Im)^ a circu- 

 lar (cm) and an inner longitudinal layer (^?^), together with dorso- 

 ventral and radial muscles. Of these, the outer longitudinal layer is 

 the most massive and maintains its relative thickness throughout the 

 length of the body. 



Broad bands of radial fibres (fig. 1, rf) from the nervous plexus 

 and inner layers of the body-wall divide this layer radially into a 

 multitude of thin, muscular bands. After passing through the outer 

 longitudinal muscular layer the radial fibres, in part, spread out to 

 form the loose framework of the cutis, described aboA^e ; the rest 

 pass to the epithelium. The circular muscular layer is also very 

 thick in the oesophagal region, but becomes relatively weak towards 

 the end of the body. It averages about one-half the thickness of 

 the outer longitudinal layer and extends forward as far as the brain 

 commissures. 



The inner longitudinal layer is much weaker than either of the 

 two outer layers. Radial fibres from the nerve-jDlexus pass through 

 this, as well as the circular layer, to innervate the alimentary canal, 

 jDroboscis sheath, and other organs. The muscles to these organs and 

 the dorso-ventral muscles also pass through this layer to become 

 attached in the circular layer. 



In this species the intestinal region is very flat in order to adapt 

 it for swimming, and the dorso-ventral muscles (Plate XIV, fig. 8, 

 dnx) are excessively developed in this part of the body. These mus- 

 cles are entirely wanting in the anterior oesophagal region, although 

 they occur in small bundles farther back at the lateral borders of the 

 oesophagus and increase in strength towards the intestinal region, 

 where they form broad, double bands alternating with the intestinal 

 caeca (Plate XIV, figs. 8, 10, 11). These bands form the anterior 

 and i^osterior walls of the genital sacs. The tw^o bands between 

 each caecum are united near the intestinal axis and are much thicker 

 here than distally. The dorso-ventral muscles spread out and ter- 

 minate, above and below, in the circular muscular layer. 



The muscular layers are composed of muscle-bundles (Plate XV, 



