12 LEPADIDiE. 



not possess any tropin. In Ibla (in which the larva is 

 large enough for dissection), the base of the proboscis 

 arises posteriorly to the first pair of legs, and the orifice 

 at the other end reaches beyond or posteriorly to the 

 point, where the mouth in Scalpellum opens, namely be- 

 tween the middle pair of legs. The mouth being either 

 so largely probosciformed or seated only on a slight 

 eminence, in two genera so closely allied as Ibla and 

 Scalpellum, and (judging from Mr. Thompson's figures, 

 and from what I have seen myself,) in the species of the 

 same genus Lepas, is a singular difference : in the cases in 

 which, at first, the proboscis is absent, it would probably 

 soon be developed. I cannot but suppose that the in- 

 wardly directed spines on the bases of the two posterior 

 legs, which are so rapidly developed, serve some important 

 end, namely, as organs of prehension for the larvae, like the 

 mandibles and maxillae of mature Cirripedes, for seizing 

 their prey, and conveying it to their moveable mouths, 

 conveniently seated for this purpose. 



The first pair of legs answers, as I believe from reasons 

 hereafter to be assigned, to the outer pair of maxillipods 

 in the higher Crustacea ; and the other four legs to the 

 first two pair of thoracic limbs in these same Crustacea ; 

 this being the case, the highly remarkable position of the 

 mouth in the larva, either between the bases of the two 

 posterior pair of legs, or at least posteriorly to the first 

 pair, together with the probable functions of the spiny 

 points springing from the basal segments of the two 

 hinder pair of true thoracic limbs, forcibly bring to mind 

 the anomalous structure of the mouth being situated in 

 the middle of the under side of the thorax, in Limulus, 

 — that most ancient of crustaceans, and therefore one 

 likely to exhibit a structure now embryonic in other orders. 

 I will only further remark, that I suspect that the trun- 

 cation of the anterior end of the carapace, has been 

 effected by the segments having been driven inwards, 

 and consequently, that the larger antennae within the 

 lateral horns, though standing more in front than the 



