EDR I OP II Til A L MIA . 



69 



Edriophthalmia have a direct development without metamorphoses, hatching from 

 the egg in the perfect form. There is usually a partial segmentation of the egg, fol- 

 lowed by the formation of an embryonic disc, and the subsequent out-buddino- of 

 the limbs. In connection with the embryonic state there is developed on the dorsal 

 portion of the body a peculiar structure, the so-called micropyle, or dorsal organ, the 

 functions or signification of which are entirely unknown. 



The Edriophthalmia are divided into two well-marked orders : Isopoda and Am- 

 phipoda. 



ORDER I. --ISOPODA. 



The name of this order signifies equal feet, and was given from the apparent simi- 

 larity of the locomotive members. Strictly 'speaking, it does not fully apply to all of the 

 individuals of the group, for in several forms (for example, Astacilla, from our own 

 shores) the walking-feet are not so uniform as the name would indicate. The Isopoda 

 are usually flattened (depressed) forms, occurring either in the sea, in fresh water, or 

 on the land. 



The antennulai lack the secondary flagellum so common among the Decapods ; the 

 mouth parts, four in number, are strong and well developed. The legs are, for the 

 most part, small, and more or less uniform in shape, 

 terminating in hook-shaped claws, a chelate condition 

 being comparatively rare. From this uniform struc- 

 ture of the ambulatory members variations may be 

 noticed in many directions, to which we shall recur 

 in treating of the various families. In the female 

 Isopoda the legs bear at the base a series of mem- 

 branous plates, each extending horizontally toward 

 the leg of the other side, thus forming, with the 

 under surface of the body, a brood-pouch, in which the eggs undergo their incuba- 

 tion. 



The abdomen is well developed, but the segments of which it is composed are fre- 

 quently fused together to a greater or less extent, all being distinct, for instance, in 

 Lygia, while in Asellus the sutures have almost entirely disappeared. The first five 

 pairs of abdominal limbs play no part in locomotion, but are developed into broad 

 foliaceous membranous plates, specialized for respiratory purposes. The last pair 

 always differ from the others, and either form a cover to protect the respiratory plates 

 closing over them like folding-doors, or, as is rather more usual, retain more or less of 

 the functions and structure of the other locomotive limbs. 



In internal anatomy we have nearly the typical structure of the Crustacea, the vari- 

 ations being comparatively few, and unimportant. The development is direct, the 

 young hatching in nearly the form of the adult, or occasionally 

 with a smaller number of segments and appendages. 



The Isopoda are, to a large extent, carnivorous, living almost 

 wholly upon animal substances. Among them we find a number 

 of parasitic forms, which live on the skin, among the gills, or in 

 the mouth of fishes, while others affect other Crustacea. The 

 greatest number are found in the sea, where the service they do 

 in consuming decaying animal matter is inestimable. The fresh- FIG. ss. 

 water forms are less numerous, while the whole family Oniscidre, or farged" e 



FIG. 87. Transverse section of Parcel iio, 

 showing brood-pouch, a. Plates en- 

 closing c, brood cavity. I. Legs. 



