168 LECTURE XIV. 



when the parts of the cast-off shell frequently return so nearly to their 

 old positions as to represent the outward form of the animal with all 

 its appendages, even the hairs and lining of the stomach. In one or 

 two days the calcification of the new crust is completed, and the animal 

 is restored to health and activity. During this period two very re- 

 markable accumulations of calcareous matter, situated at the sides of 

 the stomach, and known in the old pharmaceutical works as " Oculi 

 Cancrorum," have disappeared : it would seem that the hardening 

 material had been previously accumulated in readiness for the rapid 

 calcification of the new crust, in order to reduce to the shortest period 

 the defenceless state of the craw-fish after its moult. 



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 vermiform, isopodous, and isocyclous Crustacea, the 

 dermal skeleton has become sufficiently 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 Anellides, we obtain most instructive tests of 

 the value of that hypothesis which ascribes sensation exclusively to 

 the ganglions, and the motive energy to the non-ganglionic nervous 

 columns of the Articulata. The same hypothesis will be more 

 severely tried by the comparative anatomy of the nervous system in 

 the higher species of the Crustacea, on account of the varying con- 

 ditions as to sensation and motion which different parts of their more 

 diversified forms of body present. 



We find in the lowest Isopoda, as the woodlouse (Oniscus) and 

 the sandhopper (Talitrus), that the supposed sensitive organs, the 

 ganglions of the abdominal chords, are more developed than in the 

 highest Anellides : they are likewise more distinctly bilobed ; each 

 lateral chord presenting its own ganglionic enlargements, which are 

 in juxtaposition, but not confluent, so that there is a distinct pair of 

 ganglia for each segment. If these ganglions be microscopically 

 examined, three orders of nervous filaments may be distinctly per- 

 ceived in them. The first is longitudinal, and extends over the 

 dorsal aspect of the ganglia; the second is also longitudinal, and 

 originates from, and terminates in, the ganglia ; the third is trans- 



