CLASSIFICATION OF CRUSTACEA. 14Q3 



Ninth. The eyes and antennas have separate segments, and the 

 abdomen is very long and large. 



The fifth point of degradation is seen in the Euphausidce; the sixth, 

 in the Mysidce and other Anomobranchiates ; the seventh is found in 

 several genera of the same group; the eighth in certain Mysidge. 

 The Anomobranchiates are thus degraded Macroura. There is not 

 merely a relaxing of the centralization ; but the forces are so weakened 

 as not to succeed in finishing out the members in the system of struc- 

 ture to which they pertain. The species consequently are not modi- 

 fications upon the level of the Macroural type, nor upon a distinct 

 level or distinct type; but simply imperfect developments of the 

 Macroural structure below the true level of that type. They bear 

 nearly the same relation to the Macroura, that the Anomoura bear to 

 the Brachyura. The ninth step is seen in the Squilloidea, whose 

 relaxation of system and elongation in the cephalic part, as well as 

 abdomen are remarkable. 



The continuation of the line of degradation represented in the Ano- 

 moura, is not to be found, as we have remarked, among the typical 

 Macroura. But the structure of the Paguri may be traced into the 

 aberrant Macroura, called Thalassinidea; and thence, both in the 

 abdomen, the legs, and the branchiae, we observe a transition to the 

 Squilloids, one division of the Anomobranchiates. If then, we were 

 to trace out the lines of affinity in the species, it would be from the 

 Mysis group to the typical Macroura, and from the Squilla group to 

 the Thalassinidea, as elsewhere explained. From the latter, the 

 lines lead mainly to the Anomoura and higher species. 



In our review, thus far, we recognise one only of the primary types 

 of structure among Crustacea. This primary type is characterized by 

 having nine normal annuli or segments devoted to the senses and 

 mouth, that is, to the cephalic portion of the body. It includes two, 

 or, we perhaps may say, three secondary types. The first of these 

 secondary types is the Brachyural; it has the antennas small, the 

 inner pair in fossettes, the abdomen without appendages. In the other 

 type (or other two, if so considered), the antennas are elongated, and 

 both pairs free, the abdomen is elongated, and furnished with a series of 

 appendages. This, the second type, is the Macroural; or, if we 

 assume that it embraces two distinct types (a second and third), the 

 two correspond to the typical Macroura and the Thalassinidea. 



