xxxvi INTRODUCTION. 



branchial appendages and other parts of the system that 

 from their transparency and tenuity may be conveniently 

 examined, afford presumptive evidence against the circula- 

 tion of the blood being confined to walled channels. 



In the Isopoda, the branchial organs are variously diffe- 

 rentiated. In some, as Ligia, for example, the passage of 

 the circulating fluid through the branchial plates is clearly 

 and distinctly defined (Fig. 12). The main artery, com- 

 mencing at the base, gives off numerous lateral branches, 

 that divide and sub-divide into a rich plexus with abundant 

 capillary vessels. In the genus Spharoma, the branchial 

 organs consist of a series of plates attached 

 to the posterior wall of the fourth and 

 fifth pairs of pleopoda (Fig. 13). In the 

 degraded family of the Bopyridce, the bran- 

 chial organs are depauperated to the lowest 

 degree, being in some genera little more 

 FIG. is. than excrescences on the ventro-lateral 

 margins of the pleon. 



In Tanais, the true branchiae have not been clearly 

 determined. It is the opinion of Dr. Fritz Miiller, Van 



Beneden, and Doctor Anton 

 Dohrn, that an appendage 

 attached to the first pair of 

 gnathopoda is not a branchial 

 organ, but a flabelliform ap- 

 pendage, that by its constant 



and unvarying motion induces the surrounding medium 

 to flow over the branchial appendages that as yet have not 

 been discerned. 



At page 122 of the second volume of this work we 

 have described and figured one of the pereiopoda with a 

 sac-like appendage attached, that we considered as the 

 homologue of the branchial sac in the normal Amphipoda. 



