498 



CRUSTACEA. 



and Mysis groups, a grade of species inferior to the Macroura. The 

 following are the only points in these distinctions which it is neces- 

 sary now to consider. In the Macroural type : — 



A. The branchiae are thoracic, and are covered by the carapax 

 [instead of being uncovered, and sometimes attached to the abdominal 

 appendages, or wholly wanting, as in the Anomobranchiates]. 



B. The abdomen is either shorter or but little longer than the 

 thorax [instead of being very much longer, as in S'quilla]. 



C. The legs are simple [instead of being two-branched]. 



We may now apply these distinctive characters in deducing the 

 natural subdivisions of the Macroura. 



a. A free extended abdomen, with pairs of natatory appendages 

 below, and caudal appendages behind; inner antennae without fos- 

 settes, and vulvae in the base of the third pair of legs, characterize all 

 the Macroura without exception. 



b. The lateral suture of the carapax, strongly marked in the 

 Brachyura, is almost wholly confined to Brachyural species, and to 

 the few Anomoural. De Haan makes the absence of this suture an 

 invariable law for the Macroura, and, as has been stated, he has 

 transferred the Galatheidea from the Macroura to the Anomoura, on 

 this ground. But there are still a few true Macroural species which 

 have this peculiarity; these are the Thalassinidea or digging Ma- 

 croura, and they are thus widely separated from the other groups. 

 But few of the species fail of it. 



c. The bending of the carapax either side to form the under surface 

 of the cephalothorax and its union to the epistome, is one of the 

 most striking features of the Brachyura. But, while in most Ma- 

 croura the sides are free, there are a few species which approximate 

 to the Brachyura in this respect. This is prominently seen in Scyl- 

 larus, Eryon, Palinurus; also, less distinctly in Astacus and Nephrops, 

 genera that link the preceding to the more typical Macroura. We 

 observe, therefore, that here is a natural line of division among the 

 Macroura, marking off a group of superior grade — the Astacidea. 

 The importance of this distinction is brought out by De Haan. 



d. The absence of a scale from the base of the outer antennae, is. 

 without exception, characteristic of the Brachyura; while its presence 

 marks the typical Macroura. The few Macroura in which the scale 

 is wanting, hence, bear evidence in this of their higher grade. They 



