I 8 A BIOLOGY OF CRUSTACEA 



crab with only three pairs of effective walking legs apart from the 

 pincers. 



To summarise the course that the evolution of the Crustacea has 

 taken it may be said that they probably began as small creatures 

 possessing many of the characters of the Branchiopoda. The actual 

 origin of the Crustacea is obscure; they clearly originated early in 

 the fossil record, but there is no definite evidence about their 

 ancestors. The limbs and general organisation of the head of the 

 primitive Crustacea are quite different from those of the trilobites, 

 and it is probable that the latter group represents a quite separate 

 line of evolution. 



After a period of experimentation, involving variation in the 

 number of segments in the body, and the radiation of small species 

 into a wide range of habitats, the basic malacostracan plan produced 

 a highly successful group of larger creatures, culminating in the 

 evolution of the Decapoda and the production of the crabs. 



LITERATURE 



Calman, W. T. (1909). Crustacea. In Treatise of Zoology, Pt. 7, ed. 

 E. R. Lankester. London. 



Darwin, C. (1851). A monograph of the Cirripedia. 2 vols. Ray 

 Society. London. 



Gordon, I. (1957). On Spelaeogriphus, a new cavernicolous crusta- 

 cean from South Africa. Bull. Brit. Mus. (Nat. Hist.) 5: 29-47. 



Sanders, H. L. (1957). The Cephalocarida and crustacean phylo- 

 geny. Syst. Zool. 6 : 1 12-129. 



Scourfield, D. J. (1926). On a new type of crustacean from the Old 

 Red Sandstone (Rhynie chert bed, Aberdeenshire) — Lepidocaris 

 rhyniensis, gen. et sp. nov. Phil. Trans. B. 214: 153-187. 



Slewing, R. (1956). Untersuchungen zur Morphologie der Mala- 

 costraca (Crustacea). Zool. Jahrb. (Anat.) 75:39-176. 



Tiegs, O. W., & Manton, S. M. (1958). The evolution of the Arthro- 

 poda. Biol. Rev. 33 : 255-337. 



