ARTHROPODA 



207 



(v. Buddenbrock ei ah. 1954 ; Dijkgraaf, 1956). One group, the 

 Eryonidea, confined to the deep seas, are blind, the eyes being reduced 

 to stalks only. In other species the eyes are sessile, both in terrestrial 

 Isopods (such as woodlice, Fig. 201) and in pelagic Amphipods : 

 among the latter in the smaller forms the eyes may be minute 

 {CapreUa, Fig. 202), while in the larger forms they may assume 

 enormous dimensions (the " wondrous-eyed hopper," Thaumatojis 

 magna. Fig. 203). Sedentary types such as Asellus, an Isopod which 

 lives in holes, are completely blind. 



Fig. 202. — The Amphipod, Caprella 



LjyEJItl.s. 



Two ocelli are seen on the dorsal 

 surface of the head. 



Fig. 203. — The " Wondrous-eyed 

 Hopper," Thaumatops magxa. 

 The largest known hyperiid Crusta- 

 cean, found at a depth of 2,500 in., 

 with enormous compound eyes (to 

 the right) (f natural size) (after 

 Brehm). 



Euphausiid 

 Crustacean 



Asellus 



The smaller Crustaceans (branchiopods, cojDepods, ostracods, 

 cirripedes) include a vast number of types in which the active swimming 

 forms are provided with eyes, while in most sessile and j)arasitic forms 

 the organs become degenerate. They comprise four diverse and little 

 related orders : 



(a) BKANCHiOPODS — protected by a shell and provided with 4 pairs of leaf- 

 like swimming feet. They comprise tw'O groups : (1) the phvllopoda such as 

 the brine-shrimp, Artemia, which can survive even in Salt Lake, and the large 

 fresh-water Apus, of world-wide distribution, and (2) the laterally compressed 

 minute water-fleas (cladocera), Daphnia, Polyphemus and Leptodora, so abund- 

 ant in fresh water. 



(6) OSTRACODS — small laterally compressed creatures with a bivalve shell 

 and indistinct segmentation, breeding parthenogenetically. Typical examples 

 are the fresh-water Cypris and the salt-water Cypridina. 



(c) COPEPODS — elongated segmented creatures without a protective shell. 

 Typical examples are the beautiful fresh-water Cyclops and the salt-water 

 Calanus. Copepods occur in vast numbers in the seas and constitute the most 



Artemia 



Leptodora 



Calanus 



