196 JOHN TAIT. 



include the animal under group A, Tliis indicates a difficulty in 

 drawing a sharp line of distinction between group A and group B. In 

 the second place, the coagulation observable in Homarus blood (at the 

 other end of the series) approaches, in regard to its mere firmness 

 apart from the mechanism involved in its production, most nearly to 

 the coagulation seen in tlie blood of Palinurus or of Ligia. 



ZOOLOGICAL SIGNIFICANCE. 



From what has been said it will be apparent that the various 

 categories into which Crustacea fall according to the form of blood 

 coagulation observed in them do not coincide in any striking fashion 

 with the subdivisions into which they are grouped by zoologists. 

 Within the order Decapoda alone we meet with all three types of 

 coagulation. When, witliin this order, we consider the animals character- 

 ized by one given type of coagulation, we find that they are not neces- 

 sarily close allies from a morphological point of view. Thus Palaemon, 

 Homarus, and Portunus, similar as regards blood coagulation, represent 

 extremes of decapod structure ; from a blood coagulation point of 

 view, again, we should group Astacus with Palinurus rather than with 

 Homarus, an arrangement that would appeal to no morphologist ; 

 and so on. 



On the other hand, there are indications that one and the same type 

 of blood coagulation may sometimes keep constant in the members of 

 a given zoological group. The prevalence in so many isopods of 

 coagulation C, a type rare in decapods, is one example. Another is 

 the apparent universality of coagulation A among the Maiadae. 

 Then, again, among the decapods coagulation C is confined to the 

 macrurous Reptantia. 



The question, so far as it concerns the zoologist, now comes to be : 

 Do the facts above related afford any justification for utilizing the 

 physiological method of inquiry in the task of deciding upon the 

 inter-relationships of Crustacea ? It seems that while a valid 

 case for the actual applicability of this method has not been estab- 

 lished, a case has at least been established for the desirability of 

 further research along this line. As a further justification for this 

 standpoint, I would quote the following sentence from Caiman 

 (09):- 



" The classification of the Decapoda is a very difficult problem, and 

 none of the schemes hitherto proposed can be regarded as entirely 

 satisfactory. The traditional classification of the group into the long- 

 tailed Macrura and short-tailed Brachyura was established by Latreille 



