290 ZOOLOGY 



with the auditory nerve. In the watery contents of the sac 

 certain particles of sand are suspended, and act as otoliths. 

 In connection with this it is interesting to note that certain 

 genera can produce stridulating noises either, as in Alpheus, 

 by clapping together the two claws on the larger leg, or, as 

 in Palinurus, the rock-lobster, by rubbing the second joint of 

 the antennae against a portion of the carapace. 



OedebS. ARTHROSTRACA. 



Characteristics. — Malacostraca with sessile eyes. iVb cephalo- 



thoracic shield; seven, rarely fewer, free thoracic segments, 



and as many pairs of legs. Heart elongated. 



The appendages of the Arthrostraca are arranged as 



follows : two pairs of antennae, one pair of mandibles, two pairs 



of maxillae, and one pair of maxillipedes, seven pairs of free 



thoracic legs, and six pairs of abdominal legs. The free thoracic 



segments are six in number in Tanais, five in Anceios, the 



anterior having in these genera fused with the head. The 



abdomen may be reduced to a mere process, but usually it 



is fully developed, and ends in an unsegmented telson. 



The antennary gland is usually present, and opens on the 

 base of the antenna. The compound facetted eyes are always 

 sessile. The ova are usually carried about in brood-pouches 

 formed by processes borne on the thoracic legs (oostegites). 

 The young do not as a rule pass through a larval stage. 



The Arthrostraca are divided into two sub-orders, char- 

 acterised as follows : 



Sub-order 1. AMPHIPODA. 



Characteristics. — Arthrostraca with laterally compressed 

 bodies; the thoracic appendages carry the gills; abdomen 

 elongated; it bears three anterior pairs of swimming- 

 feet, and three posterior pairs of backwardly directed feet 

 adapted for jumping. 



As a rule, the Amphipods are small, but a few, some of them 

 living in Arctic seas or at great depths in the ocean, attain 

 several inches in length. They inhabit both salt and fresh 

 water, and progress by swimming or jumping. The males 



