306 COMPARATIVE ANATOMY CHAP. 



First. Apart from the unsegmented head, which carries the typical number 

 of limbs (5 pairs), the trunk always consists of two regions, the thorax and the 

 abdomen. They both have a constant number of segments, the former 8, the 

 latter 7. Each of these, with the exception of the last abdominal segment, is 

 typically provided with a pair of limbs. The limbs of the 6th abdominal segment 

 often form with the 7th or terminal segment a caudal or rowing fin. 



Second. The segments of the thorax show a tendency to fuse with the head. 

 Either the first segment or several anterior segments fuse with the head to form an 

 incomplete cephalo-thorax, or all the thoracic segments unite, and with the head 

 form a complete cephalo-thorax, which then shows external signs of the original 

 segmentation only on the ventral side. 



Arthrostraca (Amphipoda, Isopoda, and Anisopoda). The foremost thoracic 

 segment is fused with the head. Seven thoracic segments thus remain free (Fig. 

 197). The eyes are sessile (Edriophthalmata). In the Amphipoda, the body is 

 laterally compressed. The Caprellidce (Amphipoda) (Fig. 198) present many 

 important peculiarities. The 2d thoracic segment is also fused with the head, 

 so that only 6 thoracic segments are left free. The abdomen is reduced to a 

 stump. The body of the Isopoda is dorso-ventrally flattened. In a few Isopoda 

 even more thoracic segments become fused with the head. Thus in the Pranizidce 

 (Anceus) the 3d thoracic segment is comprised in the incomplete cephalo-thorax. 

 A shell-fold, though as a rule wanting in the Arthrostraca, is found in the Anisopoda, 

 although slightly developed. In these latter the 2 anterior thoracic segments 

 are fused with the head. In parasitic Isopoda the metamerism of the body may be 

 indistinct and obliterated, and the body itself become asymmetrical. 



Thoracostraca. A shell-fold is everywhere developed, proceeding from the head ; 

 this, as a cephalo-thoracic shield, fuses dorsally with the integument of a larger or 

 smaller number of thoracic segments, but, unlike the corresponding integumental fold 

 of the Entomostraca and Leptostraca, it only covers the thorax, never the extremities 

 and the abdomen. On the head (except in the Cumacea) there are 2 stalked facet- 

 eyes (Podophthalmata). 



I. Cumacea (Fig. 199). The cephalo-thoracic shield. remains small ; the cephalo- 

 thorax includes the 3 or 4 anterior thoracic segments ; the 4 or 5 posterior ones 

 remain free and distinct from one another. The abdomen is long and slender, 

 and distinctly segmented. In the female it carries no feet ; the 6th segment only 

 has on each side a pair of biramose stylet-like limbs. The two eyes (when present) 

 are fused into one unpaired eye, or are very close to one another. 



II. Stomatopoda (Fig. 200). The cephalo-thoracic shield is fused with the most 

 anterior thoracic segments and covers the thorax with the exception of its 3 

 posterior segments which remain free. The broad abdomen is very strongly 

 developed, and longer than the cephalo-thorax. The strongly developed lamellate 

 limbs of the 6th abdominal segment form with the limbless terminal segment (telson) 

 an imposing caudal plate (swimming fin). 



III. Schizopoda (Fig. 201). The soft-skinned cephalo-thoracic shield generally 

 covers the whole thorax, and fuses with the dorsal integument of a varying number of 

 its segments. One or more segments, however, always remain unfused. In the 

 Mysidce the last 5, in the Euphausidce only the last thoracic segment, remains 

 unfused. Abdomen elongated and strong, ending in a swimming fin. 



IV. Decapoda (Figs. 202 and 203). The strong cephalo-thoracic shield, which 

 occasionally becomes through calcification of its chitinous integument as hard as 

 stone, generally covers the whole thorax, and is also fused with the dorsal integu- 

 ment of the thoracic segments. A complete cephalo-thorax is thus formed. The 

 pleura of the cephalo-thorax, which project freely downwards at the sides, cover the 



