104 Descriptions of Preparations. 



whence the epipodites spring. The seven branchiferous segments, 

 which, it may be remarked, correspond with the seven segments 

 carrying ambulatory legs, in Amphipoda and Isopoda, carry ten 

 branchiae in the pairs which all of them except the first and last 

 support, and a single branchia upon the first and last of their 

 number. To these twelve respiratory organs we may add six, by 

 counting the branchiferous epipodites as such. 



In the American species of the genus Astacus, with one exception, 

 there is no branchia upon the fifth abdominal leg ; and with the absence 

 of this branchia a lesser width of the cardiac area on the carapace has 

 been observed to be correlated. In the genus Homarus, on the other 

 hand, the branchiae are much more numerous than in Astacus, and 

 indeed without counting the epipodites which, as may be seen in the 

 common lobster, are setigerous rather than branchiferous, and act mainly 

 as the scaphognathite does, by changing the water in the neighbourhood of 

 the true branchiae, they out-number all the branchial organs of the Astacus, 

 whilst the cardiac area on the carapace is relatively much less clearly 

 indicated than in the smaller Decapod. Most of the anatomical points 

 vipon which weight has been laid in these descriptions of the fresh-water 

 Crayfish, may be illustrated in the structural arrangements of the common 

 lobster, in places where the Astacus may not be procurable. The marine 

 species will however be found to diff'er fi-om the fluviatile in the following 

 points. Its hepatic coeca are shorter, and the anterior portion of the 

 entire gland is larger, whilst the vasa deferentia are shorter, the testes are 

 very long and only joined by a commissure. The coecum at the commence- 

 ment of the intestine, which has already been spoken of as the rudiment 

 of the yelk-sac, is smaller and bilobed in the lobster, whilst it is simple in 

 the Astacus. The duodenum itself is smooth, and the rectum plicated 

 internally in the lobster, and the duodenum, where it ends in the rectum, 

 has an azygos coecum appended to it dorsally, as in Amphipoda, and 

 most other Decapoda except Astacus. The nervous system differs mainly 

 in having the ganglionic mass, whence the jaws and foot-jaws ai"e inner- 

 vated, more distinctly constricted at the sides, and less in size relatively to 

 the five abdominal ganglia which succeed it, than we find the homologous 

 structures to be in the Crayfish. Of external points of difference, perhaps 

 the two most important are presented by the telson, which is uniarticulate 

 in the lobster, and the second abdominal appendage in the male, which 

 is much less modified, and differs much less in appearance from those 

 which succeed it, than the same appendage does in the male Astacus. 



