CRUSTACEA. 171 



roots of the spinal nerves in the Vertebrata have been proved to 

 possess. With respect to the anatomical grounds which were first 

 adduced in proof of this correspondence of function, they are in- 

 validated by the fact that the presence of ganglions in the sensiti\e 

 roots of the spinal nerves is not their constant character. 



The results of the experiments alluded to, though somewhat con- 

 tradictory, are, upon the whole, as might have been anticipated, 

 hostile to the conclusions founded upon a partial anatomical analogy. 

 A more extended investigation of the Comparative Anatomy of the 

 Nervous System has remedied the imperfections of the experimental 

 inquiry, has supplied the answers which were in vain attempted to be 

 gained by mutilating the living Crustacea, and has brought the hy- 

 pothesis in question to the test of deductions which may be legitimately 

 drawn from those surer experiments which Nature herself has left 

 for our instruction in the modifications of the crustaceous type of struc- 

 ture. We have here * two opposite conditions of a large and important 

 part of the trunk of two nearly allied Crustacea. In the lobster {As- 

 tacus) the abdomen or tail is encased in a series of calcareous rings, 

 forming a hard and insensible chain armour : but in the same degree 

 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 deposition 

 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 f, 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 

 monopolise 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 de- 

 rived 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, 



* Preps. Nos. 130J. and 1303. B. f Broderip, Zool. Jourii. vol. iv. p. 200. 



