CRUSTACEA. 307 



after supplying the digestive system with the stomato-gastric nerves, 

 unite below to form the ganglion which distributes nerves to the 

 maxillary apparatus and pharynx. This is succeeded by a large 

 oblong ganglion, situated at the base of the great nippers, and of the 

 second pair of feet, both of which pairs it supplies. The lateral 

 chords diverge for the passage of the artery, re-unite to form a third 

 thoracic ganglion, smaller than the second, supplying the third pair 

 of thoracic legs, and sending off three pairs of nerves posteriorly. Of 

 these the lateral pair goes to the fourth diminutive pair of feet ; the 

 median pair supplies the fifth pair of feet : the two remaining dorsal 

 nerves, which are of minute size, form the continuations of the ab- 

 dominal chords, and pass along the under or concave side of the soft, 

 membranous, and highly sensitive abdomen to the anus, anterior to 

 which the last small ganglion is situated : this supplies the nerves to 

 the muscles of the caudal plates, here converted into claspers, and 

 enabling the animal to adhere to the columella of the univalve shell, 

 which it may have selected to protect that portion of its body which 

 nature has left undefended by the usual dense and insensible crusta- 

 ceous covering. 



Experiments have been made, and repeated, in order to determine 

 whether the ganglionic portions of the abdominal chords of the 

 Articulata have the same restricted function which the posterior 

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

 possess. With respect to the anatomical grounds which were fii"St 

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

 validated by the fact that the presence of ganglions in the sensitive 

 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 miglit 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 imperfection 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 hypo- 

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

 drawn from those surer experiments w^hich Nature herself has left 

 for our instruction in the modification 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 rincs, 

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



♦ Preps. No. 1301. and 1303, B. 

 X 2 



