HOMOLOGIES OF CRUSTACEA. 4^ 



The most natural supposition, in view of the fact that the members 

 of Crustacea consist normally of three parts or branches, a tigellus, a 

 palpus, and a fouet, is that the multiplication consists in these several 

 parts (two of them or the three) becoming separate legs and at the 

 same time having separate segments in the body, the normal basal 

 portions of each possibly corresponding to these segments ; and possibly 

 we see some analogy also in the multiplication of branchias, two or 

 three being often appended to a single leg in the Decapods. 



In Limnadia, there are eighteen or twenty-seven such legs, each 

 number a multiple of three. The form of the animal, even to its ab- 

 domen and its thoracic members, is very much like a Daphnia. In 

 the genus Penilia of the Daphnia group, the number of pairs of legs 

 is six, and they occupy the sixth to the eleventh normal segments, the 

 last three segments of the thorax being obsolete, as in Caligus and Cy- 

 clops. If now the number of legs of Penilia be multiplied by three, it 

 gives the number in a species of Limnadia ; and again, if the number 

 of pairs of legs in Penilia be increased by three (the number of obso- 

 lete segments), and then the sum be multiplied by three, it gives 

 twenty-seven, as found in another Limnadia. The arrangement will 

 then be as follows : 



Normal Segment I. Obsolete. 



" " II., III. Antennae. 



" " IV. Mandibles. 



" V. One pair of maxillfe. 



VI.-XI. Six segments with eighteen pairs of branchial plates. 

 " " XII.-XIV. Three segments, obsolete. 



Or, if the number of branchial plates is twenty-seven, the normal 

 segments VI. to XIV. (nine in number) may correspond to them. 



In Nebalia* there are only the normal number to the thorax, if the 

 four pairs of two-branched or natatory members are annexed to the 

 abdomen, as so considered by Milne Edwards.f But by this arrange- 

 ment, the abdomen is abnormal in number of segments when the 



* We here take under consideration the Nebalia Geoffroyi, well figured and described 

 by Milne Edwards; see Ann. des Sci. Nat., xiii. 297, pi. 15; ibid. [2], iii. 309; also 

 Illust. of Cuv. llegne Animal, by M. Edwards, pi. 72, fig. 1. The species figured by 

 Leach, and upon which the genus was founded, has not yet been described with full 

 details ; it appears to have five pairs of natatory feet. 



f llegne Animal, Crust., pi. 4. 



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