CLASSIFICATION OF CRUSTACEA, 7 



with some modifications as to the relative importance of these divisions, 

 and the mode of grouping. The species with pedunculate eyes are 

 naturally separated from those with sessile eyes ; and, in subdividing 

 the latter, the large group including the Amphipoda and Isopoda are 

 as properly separated from the other species, or Entomostraca. These 

 steps in classification were first recognised essentially by the Swedish 

 naturalist, Linnaeus. 



The Cirripeds also have sessile eyes in the young, if not in the 

 adult, state, and might be arranged with the second of the divisions 

 mentioned. Yet they have so many peculiarities of structure, and 

 their habits are so different from those of other Crustacea, that they 

 more properly form a third grand division. Though Entomostracan 

 in the young state, they subsequently develope in a widely divergent 

 line, producing species with a persistent shelly covering not liable to 

 be thrown off like the skin of the rest of the body, and having a fixed 

 instead of a migratory body, with many peculiarities of structure. 



The three grand groups among Crustacea are then as follows : 



I. Crustacea Podophthalmia. 



II. Crustacea Edriophthalmia. 



III. Crustacea Cirripedia. 



I. Podophthalmia. — The Podophthalmia have a great similarity of 

 structure, although exceedingly diverse in form, — a diversity princi- 

 pally owing to the greater or less development of the abdomen. The 

 large carapax covering the thorax, exposing only two or three pos- 

 terior segments, if any, and the characters of the cephalic organs and 

 mouth, are very uniform features for the species. A variation takes 

 place in the number of buccal appendages, but this consists simply in 

 the posterior pairs being either appropriated exclusively to the mouth, 

 or being so elongated as to act the part of feet. 



There are species, however, which are removed from the rest by 

 characters of high importance ; yet such species are only examples 

 of inferior development; — that is, they are analogous in general 

 character to the condition which the typical species present before 

 arriving at complete maturity. Their degradation is seen in their 

 having the thoracic branchiae exposed, instead of covered by the 

 carapax ; and, in a lower stage, in having no thoracic branchiae but 

 only similar appendages to the abdomen ; and, in a stage still lower, in 

 the branchiae being wanting altogether, and even the abdominal appen- 

 dages rudimentary, as well as one, two, or three posterior pairs of tho- 



