LARVAE OF DECAPOD CRUSTACEA 339 



CONCLUSION 



While no doubt has been expressed as to the vaHdity of the Thalassinidea as a group, 

 there have been differences of opinion as to its systematic position. The Axiidae, 

 lacking the linea thalassinica, and with, in some cases, an almost complete series of 

 gills and other primitive characters, so closely approach the Nephropsidea that Bouvier, 

 for example, has gone so far as to describe Calocaris as an "Homaride fouisseur", and 

 to say "aucun de ses caracteres si ce n'est I'absence de pinces sur les pattes de la 

 troisieme pair, ne permet de distinguer les Thalassinidea des Homaridea." If it is 

 somewhat extravagant to say this of the Thalassinidea as a whole, it is more or less true 

 of the Axiidae alone and this close relationship was recognized by Claus (1885, p. 59). 

 Beurlen (1930, p. 386) emphasizes the primitive characters of the Axiidae, which go 

 back to the Jurassic, and suggests derivation of Callianassidae from them through the 

 Cretaceous Protocallianassa. Ortmann and Boas also regarded the whole group as 

 closely related to the Homaridea, but Borradaile and Caiman treat them as Anomura. 

 Bouvier dismisses those characters which seem to indicate Anomuran affinities as due 

 to evolution towards a burrowing habit. 



It is necessary to determine what light, if any, the larvae throw upon the question. 



Taking the Thalassinidea as a whole it is impossible to frame any definition for the 

 larvae which is at all precise. The following summary of characters common to all is 

 subject to so many exceptions that it can hardly be called a definition. 



(i) Abdomen commonly with median dorsal spines. 



(2) Telson usually with median spine. 



(3) Spine 2 of telson reduced to a hair in early stages. 



(4) Endopod of antenna normally with three apical setae. 



(5) Exopods, with rare exceptions, on three or more legs. 



(6) Pleopods absent from somite i. 



All these characters are also found in Homariis, and point to a close relation with 

 Nephropsidea. On the other hand closer analysis of the larval characters of the three 

 families about which we have some knowledge leads to the conclusion that the Thalas- 

 sinidea are not a homogeneous group, and I have already (1924) suggested that there 

 are two series of genera which may be regarded as Homarine and Anomuran re- 

 spectively. 



While the larvae of Axius stirhynchiis and Callianassa subterranea present many 

 obvious differences, there is very much less difference between the former and C. 

 laticaiida, and we know now the larvae of undoubted Callianassa which still further 

 close the gap between them (Gurney, 1937). As has been shown above there are many 

 larvae which I have assigned to Axiidae and Callianassidae which form an unbroken 

 series between C. subterranea and larvae of unknown species which must belong to 

 Axiidae though they differ so much from Axius stirhynchus. 



The larvae of Axiidae and Callianassidae cannot by any means be distinguished, 



