336 



CRUSTACEA. 



trunks (h, h, h), which ramify through the individual branchise, and 

 supply all their appended filaments. Having undergone exposure to 



the respired medium, the blood is again collected from the branchiae 

 by branchial veins (&, k, k) represented on the opposite side of 

 the body, and conveyed by the large vessel, /, to the dorsal sinus 

 (Jig. 158, 5), where, being mixed up with the general mass of 

 blood contained in the sinus, the circulating fluid is admitted into 

 the heart through the valvular orifices (d, d), to recommence the 

 same track. 



(372.) As might be anticipated from an examination of the ex- 

 ternal configuration of the different families comprised in the exten- 

 sive class we are now considering, the nervous system is found to 

 pass through all those gradations of developement which we have 

 found gradually to present themselves as we have traced the Homo- 

 gangliata from the lowest to the most highly organized types of 

 structure. In the most imperfect Crustacea, indeed, we find a 

 simplicity of arrangement greater than any hitherto pointed out 

 even in the humblest Annelida ; a disposition of parts which the- 

 oretically might have been expected to exist, but has only been 

 distinctly recognised in the class before us. 



We have all along spoken of the nervous centres of the Arti- 

 culata as arranged in symmetrical pairs, although in no example 



