286 ZOOLOGY 



but are better developed in the male ; the sixth is flattened and 

 lamelUform, and assists the telson in forming the swimming 

 caudal fin. In Mysis 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. Uuphausia is 

 further remarkable amongst the Malacostraca for having a 

 Hauplius 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 iy the carapace. The 

 posterior five pairs of thoracic limhs 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), etc. 



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 tvn 

 pairs of feet ; in the female it is broader, and has four pairs 

 of appendages. 



