94 MUSCULAR SYSTEM. 



For retraction, however, there are two muscles, a h 

 and G d'. The first is attached to the extreme end of 

 the filament (which in PL LXII, fig. 12, is represented 

 as partially retracted) ; it passes all along the filament, 

 and then close to the walls of the body, between them 

 and four lateral muscles, which tend to keep it in 

 place, and is then attached to the back, not far from 

 m, and near the median line. The other muscle {c d') 

 is attached to the middle of the filament ; it runs 

 parallel to the preceding, also passes between the four 

 lateral muscles and the skin, and divides into two 

 ends, which terminate near one another, and still 

 close to the central line of the body. In Pis. LVII 

 and LYIII the same parts are seen during retrac- 

 tion. 



The presence of two muscles instead of one is neces- 

 sary, owing to the length of the organ. It is evident 

 that by itself the muscle a h would be able only 

 partially to withdraw the filament, the length of the 

 latter being greater than the distance from the inser- 

 tion of the muscle to the end of the ventral tube, e ; 

 the terminal portion of the filament would therefore 

 still be left outside, if it were not for the muscle c d'. 

 Moreover, when the organ is about to be protruded, it 

 is probable that the muscle c d' relaxes first, and thus 

 the filament passes out regularly, whilst, if there were 

 only one muscle, it might from its length, get into a 

 knot. 



I will now pass to the muscles of the linear species, 

 taking Tomocerus as the type, and will more particu- 

 larly describe those of the 3rd and 4th abdominal 

 segments, as these are specially concerned in the 

 mechanism of the spring. They are represented in 

 Pis. LIX, LX, and LXI, the same muscles being 

 indicated in all three by the same letters. In Pis. 

 LIX, LX, a side view is given ; PI. LXI gives the 

 ventral surface ; in PL LX many of the muscles 

 represented in PL LIX are cut away, so as to show 

 those lying underneath, i. e. outside them. I have 



