14 A BIOLOGY OF CRUSTACEA 



The presence, in the adult female, of a hrood pouch under the 

 thorax is the hall-mark of the Peracarida, various members of 

 which are shown in fig. 14. A well-developed carapace is present 

 in the Mysidacea, and a less well-developed one in the Cumacea, 

 Tanaidacea and Spelaeogriphacea. In the last group the carapace 

 only covers the first thoracic segment; in the tanaids it is joined to 

 the first two thoracic segments, while in the Cumacea it is joined to 

 the first three or four segments of the thorax. The isopods and 

 amphipods do not have a carapace. It is in these two groups that 

 the greatest radiation has occurred. As a general rule it may be 



Fig. 14. An evolutionary tree of the Peracarida. Based on the 

 ideas of Siewing as modified by Gordon (1957). 



said that the amphipods are flattened from side to side while the 

 isopods have flat backs, but there are various exceptions to this 

 rule. The best way of separating the two groups is to examine 

 the structure of the abdominal appendages. In the isopods they are 

 flattened and used for respiration, while in the amphipods they 

 are usually narrow and branched. 



The question mark by the line leading to the Thermosbaenacea 

 in fig. 13 draws attention to the uncertainty of the systematic 

 position of these small creatures. Their anatomy is peculiar in 



