4(52 BULLETIN OF THE BUREAU OF FISHERIES. 



line. The inner and outer plates are formed by the anterior expansion of the second 

 and third joints, respectively; the remaining joints, four in number or less, con- 

 stitute the palp. Both inner and outer plates are frequently furnished with stout 

 spines, which are employed in mastication. 



Thoracic legs. — The first two pairs of thoracic legs differ considerably in 

 structure from the succeeding appendages and are designated gnathopods. The 

 remaining five pairs are called the perseopods. Each thoracic appendage consists 

 of seven joints, which may be designated, counting from the articulation with the 

 body, as the coxal plate, basal joint, ischium, merus, carpus, propodus, and dactyl. 

 The first joint or coxal plate is joined so as to permit only a small amount of lateral 

 movement, and lies mainly outside the following joints, so that it apparently does 

 not form a part of the appendage. The basal joint is elongated. The ischium, 

 except in the posterior gnathopods of the Lysianassidse, is short. The three follow- 

 ing joints vary greatly in their relative development in the different groups. The 

 terminal joint or dactyl is usually in the form of a claw. Only very rarely is it 

 absent (Haustorius, Bathyporeia). The gnathopods usually have the propodus in 

 the form of a hand, and are adapted for grasping objects, although in many forms 

 they are also employed in ordinary locomotion. The structure and relative size of 

 the gnathopods vary exceedingly in different groups. In some cases the dactyl 

 closes against a thumb-like process of the hand, as in the claw of the lobster, and in 

 such cases the gnathopods are said to be chelate. Usually the dactyl closes against 

 one margin of the hand, the palm, and then the gnathopods are said to be subchelate. 

 Marked sexual differences are common in structure as well as in the size of the 

 gnathopods, and in several species (Jassa, some Orchestias) a dimorphism occurs in 

 the second gnathopods of the male. The genus Batea i> unique in having the first 

 gnathopods in a rudimentary form. 



Perseopods. —The first two pairs of peraeopods are usually of similar form and 

 nearly equal size. They are generally smaller and less stout than the following pairs 

 and have a narrow basal joint. Their coxal plates, like those of the gnathopods, 

 are generally large. The dactyls in nearly all amphipods point backward. In many 

 genera which Delia Valle unites under the family "Corofidi" the first two pairs of 

 perseopods contain glands which may extend from the second into the fifth joint and 

 which produce a sticky fluid which is discharged through a duct opening at the tip 

 of the dactyl. This fluid, which hardens into a sort of web as it is drawn out of the 

 duct, is used in the construction of tubes or nests in which the animal takes up its 

 abode. The following three pairs of perseopods usually have small coxal plates and 

 broad basal joints. They are generally of unequal size and in many genera are very 

 dissimilar in form. The dactyls usually point forward. 



Abdominal appt m.dages. — The abdominal appendages of amphipods fall under 

 two very different types. The anterior three pairs, the pleopods, are adapted for 

 swimming. Each consists of a single basal piece which bears two multiarticular 

 rami, which are furnished with long, plumose seta' on both sides of each joint. The 

 two basal pieces of each pair are held together by a series of coupling spines on the 

 lower portion of the inner margin. The three posterior pairs of appendages, or the 

 uropods, are firm in texture and comparatively immobile. They all point posteriorly 

 and arc closely approximated. Each consists of a basal piece., or peduncle, and two 



