CRUSTACEA. 
273 
find anterior to the pleou. In the pleon all the somites are de- 
veloped except the antepenultimate, which is afterwards pro- 
duced by a division of the nltimatc. 
Now we contend that the newly-developed organs mark the 
separation between the different parts of the animal, and that 
each consists of seven somites. To this the insect-type would 
bear a very close analogy, if we suppose that the lessened 
number of somites is due to the suppression of the third and 
seventh of the cephalon, and the two first and two last in the 
pereion, of the typical Isopod. 
We certainly think that, before the homological relation of 
the several parts of a crustacean can be conclusively demon- 
strated in the insect, there should be a uniformity of opinion 
among earcinologists as to the homologies of the different orders 
of Crustacea. 
M. Cii. IlouGET states, in a note (^Comptes Rendus,^ vol. lix. 
p. 8.51 ), that the similarity of the mode of the termination of the 
motor nerves which M. Kuhn thought he had established in 
the Artieulata, and which had long since been indicated by 
M. Doyere, with that which M. Rouget had himself demon- 
strated to exist among the superior Vertebrata, was contrary to 
the most direct observation. M. Rouget then sets forth the 
result of his observations on the termination of the motor nerves 
among Crustacea {Canc.cr {Carcimts?) mccnaSy Astacus), as well 
as among some of the larv?e of Dipterous and Coleopterous insects. 
He states that the terminal cone described by Doyere among 
the Tardigrades, and by M. Quatrefages among the Eolidines, 
truly exists, but that it is not itself the true termination of 
the nerve. After detailing shortly his researches, he states that 
there is no trace of a terminal organization analogous to that 
which he has shown to exist in the superior classes of Ver- 
tebrates ; but, on the contrary, there is much resemblance be- 
tween the mode of termination of the fibres of the motor nerves 
of the Artieulata with that which he has described as existing 
among the Batrachians. 
It results, he says, from his observations, that there is no 
similarity between the termination of the motor nerves of the 
higher Vertebrates and those of the Artieulata. For whilst 
among the Artieulata and the inferior Vertebrates the ^^cylinder- 
axis presents not any modification of appearance or structure 
at its terminal extremity, in the three superior classes of Verte- 
brates the cylinder -axis spreads out in the form of a finely 
granulated plate, accompanied by a special agglomeration of 
plasmatic nuclei. One disposition alone is common to all 
these modes of termination. Moreover the essential element of 
the nervous fibre penetrates through the sarcolemma until it 
reaches a bundle of muscular fibres, and the substance of the 
1804. [vol. I.] T 
