VI ARACHNIDA, CRUSTACEA 259 



costraca — including the larger and more familiar types such as Lobsters, 

 Crabs, Sand-hoppers, and " Slaters " — and the Entomostraca, constituted 

 by less conspicuous creatures smaller in size and simpler in structure. 

 It has, not unnaturally, been customary to regard the Entomostraca as 

 representing an earlier phase in the evolution of the Crustacea, but there 

 is much to be said for the contrary view which would regard their simpler 

 structure as being a secondary acquirement correlated with their diminu- 

 tion in size. A strong argument in favour of this latter view is that modern 

 investigations have shown a very striking agreement between the Mala- 

 costraca, the Insecta, and the Arachnida, in the number of segments of 

 which the body is built up. There is no agreement between the different 

 species of Peripaius or of the Myriapoda as to the number of body 

 segments, so these animals represent an intermediate phase in evolution 

 between the Annelid with its indefinite number of segments, and the 

 Arachnid or Insect with its fixed number of segments. If the Mala- 

 costraca had arrived at their definite number of segments by an 

 independent evolutionary path it seems improbable that the resulting 

 ' number of segments would agree with that arrived at by the Arachnids 

 and Insects. It is therefore tempting to lean towards the, admittedly 

 heretical, view which would regard Insects, Arachnids, and Malacostracous 

 Crustaceans as being all of them modern representatives of a common 

 ancestral type of arthropod in which the number of body-segments had 

 become definite. The Entomostraca, in which the number of segments 

 shows great variation, would be regarded not as having retained the 

 primitive indefiniteness but as having secondarily lost the constancy in 

 number of segments possessed by the ancestors common to them and the 

 other Arthropods. 



The most familiar members of the Malacostraca are those which 

 are grouped together under the name Decapoda — including the Lobsters, 

 Shrimps, Prawns, Hermit-crabs, and Crabs. They are characterized by 

 the possession of a flap-like outgrowth of the body-wall (caiapace) which 

 grows back from just behind the head and covers in the entire thoracic 

 region with its gills on each side. They possess the array of appendages 

 already described for the Crayfish. They usually pass, during the earlier 

 stages of their life-history, through a characteristic larval stage known as 

 the zoaea, provided with long spine-like outgrowths from the head and 

 carapace (Fig. 108). 



In certain of the Decapods interesting and characteristic modifications 

 of the abdominal region are seen. In the Hermit-crab this portion of 

 the body is thrust into an empty Gasteropod (p. 267) shell and in correla- 

 tion with this its cuticle has reverted to a soft membranous condition, 



