v AMPHIPODA CREVETTINA 137 



eluding a vast assemblage of very similar animals, of which the 

 common Gammarus and Orchestic, may serve as examples ; (ii.) 

 the Lsemodipoda or Caprellids, and (iii.) the Hyperina. 



We cannot do more than touch on the organisation of these 

 sub-orders. 



Sub- Order 1. Crevettina. 



In this sub-order only one thoracic segment is fused with the 

 head ; the basal joints of the thoracic limbs are expanded to form 

 broad lateral plates, and the abdomen is well developed, with six 

 pairs of pleopods, the last three pairs being always turned back- 

 wards, and stiffened to act as uropods. 



This group has numerous fresh -water representatives, e.g. 

 Gammarus of several species, the blind well-shrimp Nipha/rgus, and 

 the S. American Hyalella ; but the vast majority of the species 

 are marine, and are found especially in the littoral zone wherever 

 the rocks are covered with a rich growth of algae, Polyxoa, etc. 

 The Talitridae or " Sand-hoppers " have deserted the waters and 

 live entirely in the sand and under rocks on the shore, and one 

 common European species, Orchestia gammardlus, penetrates far 

 inland, and may be found in gardens where the soil is moist 

 many miles from the sea. 



The Rev. T. E. E. Stebbing, in his standard work l on this 

 group, recognises forty-one families, and more than 1000 species, 

 so that we can only mention a few of the families, many of 

 which, indeed, differ from one another in small characters. 



Fam. Lysianassidae. The first joint of the first antenna is 

 short, with an accessory flagellurn. Mandible with a palp, and 

 with an almost smooth cutting edge. The third joint of the 

 second gnathopod is elongated. This family is entirely marine, 

 comprising forty-eight genera, with species distributed in all seas. 

 One genus, Pscudalibrotus, inhabits the brackish water of the 

 Caspian Sea. Lysianss has several common British and 

 Mediterranean species. 



Fam. Haustoriidae. The members of this family are 

 specially adapted for burrowing, the joints of the hinder thoracic 

 limbs being expanded, and furnished with spines for digging. 

 Some of the species are common on the British coasts, e.g. 

 Haustorius arenarius. Pontoporeia has an interesting distribu- 

 1 "Das Tierreich," 21, Amphipoda Gammaridea, 1906. 



