214 ELEMENTARY BIOLOGY. [CHAP. 



c. The depressor muscles. Three sets are obvious — 



a. Two dehcate strips, arising from the cardiac sac, 

 immediately above the gullet. They pass ob- 

 liquely forwards and downwards, converging as 

 they do so, to be attached to the sternal skeleton 

 immediately in front of the labrum. 



fi. A small slip, arising from the extreme posterior 

 wall of the cardiac sac, and passing downwards 

 and inwards to unite with its fellow of the oppo- 

 site side, prior to its attachment to the anterior 

 end of the endophragmal system. 



y. A long slip, arising from the ventral wall of 

 the pyloric sac, and passing between yS obliquely 

 forwards and outwards. It is inserted close to 

 the base of the gullet. 



d. The dilator muscles ; small fan-shaped tracts, arising 

 from the gullet and attached — 



a. the anterior ones, to the adjacent exoskeleton ; 



/?. the posterior, to the endophragmal system. 



The upper anterior one may receive a depressor 

 slip. 



The intrinsic muscles. Carefully dissect off the le- 

 vator and depressor fibres described above and ex- 

 amine — 



e. The great constrictor ; a large sheet investing the 

 postero-ventral half of the cardiac sac (a seeker can 

 be readily inserted beneath it). Its fibres are seen 

 to be interrupted by the intervention of the postero- 

 ventral ossicles (3. b) to which they are attached. 



/. The lesser constrictors ; of these there are two sets. 



