2384 CRUSTACEA. 



1. The trunk, with the mouth aperture at its extremity. 



2. The ophthalmic segment, as he names it (" Augenring") or 

 cephalic, as we should call it, bearing 1, above, the eyes, four in number 

 coalesced on a spot of pigment ; 2, below, three pairs of appendages, 

 some or all of which may be obsolete. 



These appendages are, first, a jointed organ, usually stout and elon- 

 gate, and generally subchelate, arising from the anterior part of the 

 segment; second, a more slender palpiform organ, five- to nine-jointed, 

 adjoining the preceding, and generally considered a palpus to the 

 same normal pair of organs; third, another pair of jointed organs, 

 folding up below the body, proceeding from the posterior and inferior 

 part of the segment, and occurring usually in both sexes, according to 

 Kroyer, though obsolete in some genera : these organs are seven- to 

 eleven-jointed, and smaller than the following pairs of legs, and end 

 commonly in a claw. 



3. 4, 5, 6, are transverse segments, bearing each a pair of long 

 terete legs, consisting of nine joints, the last being a claw. 



7. A caudal appendage, which in a single genus, Zetes, Kroyer, 

 consists of two distinct segments, the first with several setse at apex. 



The homologies of these parts have been variously interpreted by 

 different authors. The trunk is described as the head, by Milne Ed- 

 wards,* and the following four segments as the thorax, the first of 

 these segments, including both the ophthalmic and the first leg-bear- 

 ing segment behind it, the two being generally imperfectly separated. 

 Kroyer calls the trunk and ophthalmic ring together the head, and 

 the following four leg-bearing segments the thorax. Johnston de- 

 scribes the ophthalmic segment as the proper head, and the trunk as 

 an appendage to it or proboscis. 



Of these views, that of Dr. Johnston appears to be most correct. 

 For the elucidation of the subject of homologies we should compare 

 the species with other sucking Crustacea. Here are their truest 

 homologues, and not among Spiders, a distinct series of Articulata. 



In the Caligoidea, we find, in the first place, a moveable trunk. 

 This trunk contains a pair of mandibles, enclosed by the transformed 

 upper and lower lips; and either side there is a single pair of maxillae 

 In Argulus, the maxillae are wanting, unless they are represented in 

 the spiculum and its sheath, a suggestion made on a former page. 



* Crust., iii. 530. 



