68 NATURAL HISTORY OF ARTHROPODS. 



SUB-CLASS IY. EDRIOPHTHALMIA. 



This group of Crustacea, which derives its name from its sessile eyes, in contradis- 

 tinction to the Podophthalmia, with the organs of vision seated upon movable stalks, is 

 also known as Tetradecapoda, or fourteen-footed Crustacea. These forms, which are 

 mostly small, possess but very slight popular interest, for but few of them are either 

 markedly injurious or beneficial, and their habits are not such as to attract much 

 attention. From a scientific standpoint they also possess but few attractions, for their 

 structure and their mode of development present but slight valuations except in 

 minute details. 



The group is characterized by a body never consisting of more than twenty seg- 

 ments, though frequently, by abortion or coalescence, even Jess than this number are 

 seemingly present. The segments which enter into the composition of the head are 

 always united, but those which correspond to the thorax of the decapods are never 

 entirely covered by a carapax ; from four to seven segments of this region being always 

 free and uncovered. The abdominal segments are frequently reduced in number. The 

 normal appendages of the body are two pairs of antennae, a pair of mandibles, two 

 pairs of maxillae, and but one pair of maxillipeds. Next in order are the seven pairs 

 of walking feet, while the abdomen bears six pairs, which are frequently adapted for 

 swimming. We here notice a difference from the Podophthalmia in the number of 

 walking feet, which is occasioned, as is readily seen, by the transfer of the last two 

 pairs of maxillipeds from the mouth parts to the ambulatory series. The eyes are 

 usually sessile, though in forms like Munna they are seated on movable stalks, or, as in 

 some other forms, they may be entirely absent. 



The respiration is usually effected by means of gills, but in some few degraded 

 forms these organs are entirely wanting. The gills, when present, are borne either 

 beneath the thorax (amphipods) or the abdomen (isopods). Among the former the 

 gills take the shape of membranous sacs, attached to the bases of some of the walking 

 feet, while in the latter the inner branch of the abdominal feet becomes modified for 

 respiratory purposes. Frequently, in the Isopods, five pairs of these gills are developed, 

 the anterior pair of abdominal feet forming a door, or operculum, which closes in the 

 others, the rest of the wall of the gill cavity being formed by the lower surface of the 



body. In the terrestrial Isopoda, however, but three pairs of 

 gills are thus developed. 



The heart is an elongated, many-chambered organ. The 

 alimentary canal is short and straight, and in some forms vessels 

 empty into the intestine near its termination. In the stomach 

 is found a triturating organ, and a straining apparatus similar 

 to that of the Podophthalmia. The liver is usually large, and 

 FIG. 86. Embryo of Gam- empties into the alimentary tract by several mouths. 



marus. m. Micropyle. x . 



.v. Yolk. b. Brain, i. The eggs, after impregnation, undergo their development in 



brood-pouches beneath the thorax, similar to those which we 



described while treating of the opossum shrimps. In the embryology we miss the 

 startling changes which we have seen in the other groups of the Crustacea, for the 



