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. 
OrpER 38. ARTHROSTRACA. 
CHARACTERISTICS.— Malacostraca with sessile eyes. No 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 Zanais, five in Anceus, 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 thoracie 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 
