92 Descriptions of Preparations. 



of perfect insects ; in some Isopoda, however, which in many- 

 points approximate to insects, they are rudimentary. The inner 

 of the two antennulary flag-ella is the smaller, and correspond to 

 the structure described in other Crustacea as a ' secondary ap- 

 pendage/ Posteriorly, and a little externally to the antennules, 

 come the antennae, each of which consists of a single multi- 

 articulate flagellum carried by a series of five basal joints, the 

 second one of which, if we count the sternal or proximal joint 

 as the first, carries the representative of the appendages which 

 are developed early upon this joint, but are subsequently aborted 

 in Carcinus maenas, in the shape of two scales, one much the 

 larger, somewhat of the shape of a short wide knife-blade, the 

 other minute, prolonged into a spinous point externally, and ar- 

 ticulating internally with the fourth basal segment. A com- 

 parison of the five basal joints of the antenna with the five basal 

 joints of such a typical seven-jointed appendage as any one of the 

 four posterior ambulatory legs in the Astacus, will leave no doubt 

 as to their correspondence ; and the multi-articulate flagellum will 

 thus come to be the homologue of the two terminal joints of those 

 appendages which are known as the 'propodite^ and ^dactylo- 

 podite,^ and are sometimes supposed to correspond to the ' tarsus' 

 of insects. The proximal joint of the antenna has a conical process 

 developed on its inferior surface, internally to the apex of which 

 there exists an orifice leading into the antennary gland. On the 

 triangular space between these two conical processes which is 

 known as the ' epistoma/ and is constituted by the sternum of 

 the antennary segment, are seen lying the anterior extremities 

 of the palpiform exopodites of the two anterior maxillipeds, which 

 correspond to the two anterior thoracic legs of insects. The pos- 

 terior pair of maxillipeds have been displaced a little backwards, so 

 as to give a better view of the two other pairs of foot-jaws, and of 

 the three pairs of jaws which they partly conceal ; though they 

 never form for them, either in Macrurous or Anomurous Decapods, 

 such a perfect operculum as they do in the Brachyurous, Posteriorly 

 to the ' epistoma,' we see the upper lip or ' labium,' and between 

 it and the first pair of thoracic appendages, or ' maxillipeds,' we 

 have the three cephalic appendages essentially concerned with the 

 prehension of food, and assisted in that function in the Hedri- 

 ophthalmatous or fourteen-footed Crustaceans by two, and here in 



