ORIGIN AND RADIATION 1 5 



having a brood pouch formed by the carapace on the back of the 

 female; no other malacostracan has a structure like this. 



At one time zoologists included the Euphausiacea and the 

 Mysidacea in a group called the Schizopoda, because the thoracic 

 legs are branched. However, the euphausids do not have a brood 

 pouch and so cannot be included in the Peracarida. They are very 

 shrimp-like, but differ from the decapod shrimps in not having the 

 first three pairs of thoracic limbs modified to form accessory mouth- 

 parts or maxillipeds. 



In many accounts of the Crustacea the Stomatopoda are left until 

 the end, as if they were a further advance on the decapods, but this 

 is by no means so. They have some primitive anatomical features, 

 such as four free thoracic segments not covered by the carapace, 

 and a long tubular heart. Their peculiarities include the highly 

 modified second thoracic limbs which resemble the front legs of a 

 praying mantis, and they have two freely articulated segments in 

 the head which bear the eyes and the antennules. In spite of their 

 superficial resemblance to lobsters the Mantis Shrimps are a 

 peculiar and quite isolated group of the Malacostraca. 



The name Decapoda implies the presence of ten legs, and this is 

 true of most decapods, though the legs are not always large and 

 functional. The first three pairs of thoracic limbs are modified as 

 mouthparts (maxillipeds) and help in the feeding mechanism, while 

 the remaining five pairs of thoracic limbs are leg-like. The first 

 pair of these legs usually bears a pair of pincers or chelae which 

 are used to catch food and for defence. 



In many ways the decapods are a most satisfactory group to 

 study from an evolutionary point of view, because the way in 

 which one type of organisation has arisen from another can easily 

 be visualised, and various types are in existence showing inter- 

 mediate stages. The type of organisation referred to is of course 

 the crab type, with its broad carapace and the abdomen tucked 

 forwards underneath. The way in which this type has arisen 

 from a creature like a lobster is not difficult to visualise when one 

 looks at Galathea and various other anomurans, even though these 

 creatures may not actually be intermediates in the line of descent. 



A primary division of the Decapoda can be made to give the sub 

 orders Natantia, the swimmers, and Reptantia, the crawlers. This 

 is not to say that some of the Reptantia do not swim— in fact 

 several of them do. The distinguishing feature is the method of 

 swimming. The Natantia use the abdominal appendages (pleopods) 



