296 COMPARATIVE ANATOMY CHAP. 



and Cryptoniscidce contain parasites which are chiefly hermaphrodite with dwarf 

 males. Body of the female deformed. Bopyrus (sexes separate), Gyge, Entoniscus, 

 Cryptoniscus. 



Order 3. Amphipoda. 



Body laterally compressed. In the typical Amphipoda only the foremost thoracic 

 segment is fused with the head, in the Caprellidce and Cyamidce the two anterior 

 segments. The gills are found on the thoracic feet as pouch-shaped epipodial 

 appendages. Where the abdomen is well developed it carries 6 pairs of biramose 

 feet, of which the 3 anterior, generally more strongly developed, serve as swimming 

 feet, the posterior, directed backwards and often stylet-shaped, as springing feet. 

 Heart in the thorax with 3, seldom 1 or 2 pairs of ostia. 



Sub-Order 1. Crevettina. 



Head and eyes small. Foot-jaws with their limb-like feelers form a large under 

 lip. Marine forms : Corophium, Talitrus, Orckestia (Fig. 197), Lysianassa. In fresh 

 water, Gammarus. 



FIG. 200. Squilla, from the side, cth, Cephalo-thoracic shield ; VI, VII, VIII, the 3 posterior 

 free thoracic segments ; 01-07, the segments of the abdomen ; a 7 , the telson ; au, eye ; a 1} anterior ; 

 02, posterior antenna ; 1-8, the 8 thoracic feet, of which 1 is the 1st foot-jaw and 2 is the 2d foot- 

 jaw or the large seizing foot, 3, 4, 5 are the posterior seizing feet, 1-5 are called oral feet, 6, 7, 8, 

 the 3 biramose rowing feet of the 3 posterior thoracic segments ; p, penis ; p\-p, the pleopoda 

 (swimming feet), p forming with the telson the powerful caudal fin ; br, the branchial tufts on 

 the exopodites of the pleopoda. 



Sub-Order 2. Hyperina. 



Head and eyes large, the latter often divided into frontal and lateral eyes. Foot 

 jaws form a small lower lip without feelers. Marine forms : Hyperia (eyes not divided), 

 Phronima, Platyscelus, Oxyceplialus. 



Sub- Order 3. Lamodipoda. 



Abdomen truncated. The 2 anterior thoracic segments fused with the head. Gills 

 on the 2d and 3d free thoracic segments ; limbs on these segments often reduced. 

 Marine forms : Caprellidce, body veiy slender and thin Caprella (Fig. 198), Proto, 

 Protella ; Cyamidce, body broad and flat, parasitic on the skin of whales, Cyamus. 



Legion III. Thoracostraca (Podophthalmata). 



With a shell fold which covers a larger or smaller part of the thorax and fuses 

 with the dorsal integument of all or some of the anterior thoracic segments, always, 

 however, projecting freely laterally and covering the respiratory cavity.as branchio- 

 stegite. A varying number of anterior thoracic segments, or all the thoracic segments 

 fused, at least dorsally, with the head to form an incomplete or a complete cephalo- 



