214 ELEMENTARY BIOLOGY. [CIIAP. 



c. The depressor viusclcs. 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. 



/?. 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 ^ obliquely 

 forwards and outwards. It is inserted close to 

 the base of the gullet. 



d. The dilator viuscles ; 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. 



