304 LECTURE XIV. 



crustaceous body at the pyloric orifice, the two levers that proceed 

 internally from the jaws to further triturate the food after it has 

 undergone the maxillary mastication, together with the whole of the 

 internal chambers of the crust, are left as entire as if dissected by a 

 skilful anatomist. 



When the common crab has thus emerged from its old shell, its 

 first feeble effort is to creep to the friendly shelter of the nearest 

 crevice. Its general appearance is that of a lump of dough, inclosed 

 in a coloured membrane. Its growth, however, at this period is 

 remarkably rapid. * 



The elementary fibre of the voluntary muscles, in the Crustacea, is 

 pale, semitransparent, but is transversely striated : the flat tendon -like 

 body into which the strong peninform muscle of the lobster's claw is 

 inserted is an internal process of the shell, and has a chitinous basis : 

 all the seeming tendons are in like manner " apodemata," or internal 

 processes, appertaining to the exoskeletal, not to the muscular, system. 

 The muscles of the trunk-segments are usually rectilinear flattened 

 masses : they are accumulated in great force in the long post-ab- 

 domen, or tail, of the lobster and other Macroura : here the flexors 

 are on the ventral, the extensors on the dorsal surface, the former 

 being the largest and most powerful. They usually pass from one- 

 segment to the next ; mostly with a longitudinal course, a few being 

 oblique and decussating. In the little Cypris, some muscular fibres 

 arising from the back of the animal act like adductor muscles on the 

 bivalve shell. 



The Crustaceous Homogangliata are not less remarkable for 

 the different conditions of their nervous system, arising out of the 

 progressive concentration of its central masses, than for the diver- 

 sity of their outward forms. Even in the higher, or Malacostracous 

 division, a very extensive series of modifications are presented, 

 which lead from the Anellidous to the Brachyurous type, or that of 

 the crab. 



In the lowest worm-like, equal-footed, and equal-ringed Crus- 

 tacea, the dermal skeleton has become suflficiently firm and resisting 

 to enable the trunk to be raised by the articulated members above 

 the ground. The muscular system attains a proportionate increase 

 of volume and force : so that when we contrast the conditions of the 

 sensitive integument and of the motor system in these Crustacea 

 with those of the same systems in the Anellids, we obtain most 

 instructive tests of the worth of that hypothesis which ascribes sen- 

 sation exclusively to the ganglions, and the motive energy to the 



* CCXXXIII. p. 8. 



