286 ZOOLOGY 
but are better developed in the male; the sixth is flattened and 
lamelliform, and assists the telson in forming the swimming 
caudal fin. In J/ysis an auditory sac is situated in the base of 
the endopodite of this last abdominal pair of limbs. In the 
family Euphausiidae there is a remarkable series of luminous 
organs, situated, as a rule, a pair on the peduncles of the eye, 
another on the basal joint of the second and seventh pairs of 
thoracic legs, and one in the middle ventral line on the first four 
abdominal segments. These organs emit at times a beautiful 
phosphorescent light, and they seem to be quite under the 
control of the animal. Their use is unknown. Huphausia is 
further remarkable amongst the Malacostraca for having a 
Nauplius stage in its ontogeny. 
Sub-order 4. DECAPODA. 
CHARACTERISTICS.— Malacostraca with usually stalked eyes. The 
thoracic segments are fused with the cephalic, and with rare 
exceptions they are entirely covered by the carapace. The 
posterior five pairs of thoracic limbs are uniramous and seven- 
jointed, and some of them are chelate. 
The Decapoda include a great number of species which 
superficially exhibit considerable differences of form and habit, 
but in essential features the group is very homogeneous. It 
may be split up into two divisions. 
Group 1. MACRURA. 
CHARACTERISTICS.—Abdomen long, four or five pairs of abdo- 
minal limbs, and a well-developed caudal fin. 
This includes the Carididae (shrimps and prawns), Astacidae 
(cray-fish and lobsters), Paguridae (hermit-crabs), ete. 
Group 2. BRACHYURA. 
CHARACTERISTICS.— Abdomen short and reduced, and bent up 
and applied to the ventral surface of the thorax. In the 
male the abdomen is narrow, and rarely bears more than two 
pairs of feet; in the female it is broader, and has four pairs 
of appendages. 
