CRUSTACEAN BLOOD COACiULATION. 197 



in 1806 ; but the difficulty of defining these groups is sliown by the 

 varying limits which have been assigned to the intermediate group of 

 Anomura established by Milne-Edwards in 1834 Boas, in 1880, was 

 the first to make a radical departure from this system. He pointed 

 out that the Brachyura and Anomura were only single branches of the 

 Decapod stock, and by no means equal in systematic value to the 

 Macrura, which included several other branches not more closely con- 

 nected with each other. In other words, just as in the classification of 

 the Malacostraca as a whole, so within the Order Decapoda, the reten- 

 tion of the primitive ' caridoid facies ' does not necessarily imply close 

 affinity between the groups exhibiting it." 



Again, referring to the Amphipoda, which were ranked by Leach 

 along with the Isopoda in his group Edriophthalmata, he says 

 (p. 239) : " It seems very likely that their affinity to the Isopoda is 

 not so close as has been supposed." 



These statements are quite in accordance with the grouping that 

 one would adopt from considerations of blood coagulation. 



PHYSIOLOGICAL EVOLUTION. 



Lastly, and this concerns principally the physiologist, we have in 

 Crustacea an exceptionally appropriate assemblage of types in which 

 to study the evolution of blood coagulation. In the blood or body- 

 fluid of all invertebrates apart from arthropods, the only form of 

 " blood coagulation " that occurs is an agglutination of the corpuscles — • 

 Cuenot (91), see also Geddes (80); there is no jellying of the plasma. 

 In arthropods alone among invertebrates we meet with a true jellying 

 or solidification of " fibrinogen " normally present in solution in the 

 plasma. Now, in the class Crustacea, and even within the order Deca- 

 poda, we find some animals whose blood does not jelly, others in which 

 the jellying process is present but insignificant, and others again in which 

 it is very conspicuous. In this group of animals, therefore, we have a 

 readily available series of types showing every gradation of evolution 

 from complete absence of jelly (e.g. the spider-crabs) to the occurrence 

 of very firm jelly-coagulation (e.g. the lobster and the rock-lobster). 



Further, in the same class of animals we find that the jellying pro- 

 cess, when present in what is possibly its most developed state, is 

 associated with the presence of corpuscles having special physiological 

 attributes. Assuming that the functional peculiarities of these cells 

 represent a high degree of selective adaptation, we have to inquire 

 by w^hat steps the specialization has been brought about. The physio- 

 logical features in question cannot be supposed to have sprung sud- 

 denly into existence in a few special animals. The explosive property 



