308 LECTUUK XIV. 



as sensibility is lost, motility is acquired ; a great proportion of the 

 muscular system of the animal is concentrated in the tail, which 

 forms its most powerful and almost exclusive organ of swimming. 

 In the hermit-crab (Pagurus), on the other hand, the muscular system 

 is almost abrogated in the long abdomen ; for this, in fact, takes no 

 share in the locomotive functions of the body : it is occupied by part 

 of the alimentary canal, and by glandular organs : the sensibility of 

 the external integument is not impaired or destroyed by the deposi- 

 tion of calcareous particles in its tissue ; but it retains the necessary 

 faculty of testing the smooth and unirritating condition of the inner 

 surface of the deserted shell, which the animal chooses for its abode : 

 minute acetabula are developed in groups upon this sensitive in- 

 tegument*, to which, also, delicate ciliated processes are attached. 

 The muscular system is reduced to a few minute fasciculi of fibres 

 regulating the action of the small terminal claspers. Now, if, as has 

 been conjectured, the ganglionic enlargements of the abdominal 

 chords monopolize the sensorial functions, and the non-ganglionic 

 tracts the motor powers, we ought to find the nerves, which supply 

 the muscles of a tail constructed almost exclusively for locomotion, to 

 be derived from non-ganglionic columns ; whilst in the tail, which is 

 almost as exclusively sensitive, the ganglions ought to have been large 

 and numerous, for the supply of nerves to the integument. The 

 contrary, however, is the fact ; six well developed ganglions dis- 

 tribute nerves to the muscular fibres of the lobster's tail ; non-gan- 

 glionic columns supply the sensitive tail of the hermit-crab. One 

 ganglion, indeed, is present in the Pagurus, but both its situation 

 and office alike militate against the hypothesis of its special subser- 

 viency to sensation : it is developed upon the end of the smooth ab- 

 dominal chords, and seems to have been called into existence solely 

 to regulate the actions of the muscles of the terminal claspers by 

 which the hermit keeps firm hold of the central pillar of his bor- 

 rowed cell. 



The general progress of the development of the nervous system in 

 the Crustacea has been, as we have seen, attended with increased 

 size, and diminished numbers of its central or ganglionic masses. 

 The divisions of each pair of ganglia first coalesce by transverse 

 approximation : distinct pairs of ganglia approximate longitudinally, 

 conjoining as usual from behind forwards : confluent groups of 

 ganglia are next found in definite parts of the body, as on the thorax 

 of those species which have special developments and uses for par- 

 ticular legs. In the crab, in which the general form of the body 



* Broderip, Zool. Joiirn. vol. iv. p. 200. 



