LARVAE OF DECAPOD CRUSTACEA 301 



I have already shown (1924) that there are larvae which do not fit into any of these 

 categories and must yet be found a place within an Axiid-Callianassid group, and many 

 more examples are given here. While it is obviously impossible to identify these larvae, 

 it should be possible to give them some sort of natural grouping and to suggest their 

 approximate systematic position. 



The salient characters available for systematic purposes are these : 

 (i) Gill formida. The gill formula should provide an excellent character, but un- 

 fortunately it cannot be entirely trusted. Gills and epipods are well developed in the 

 last stage, but, in Axius stirhynchus, the pleurobranchs do not appear even in the first 

 post-larval stage, so that, though their presence would be a very positive clue (see 

 Gurney, 1924, p. 148) their absence in the larva proves nothing. On the other hand, it 

 appears to be generally true that epipods, if present in the adult, are traceable in the 

 larva, and their presence or absence should have weight; but I do not feel entirely 

 confident that this is so. In the series of larvae dealt with here epipods are some- 

 times so well developed that they show traces of podobranchs, and these forms, 

 which also have an exopod on leg 5, can be safely excluded from the Callianassidae ; 

 but there are some without epipods which in other respects so exactly resemble 

 those that have them that a division on this ground does not seem to be justified. 

 None the less, in view of the fact that epipods are present in all Axiidae and absent from 

 all Callianassidae (except Callianidea), I have used this as a fundamental point of 

 cleavage. 



(2) Pleopods. Until recently it was possible to say that the presence of a pleopod on 

 somite 2 was a main distinguishing feature of the Axiid larva, but Callianassid larvae 

 have now been discovered which have this pleopod (Gurney, 19376). It may well be 

 that only those Callianassids with abbreviated development have four pairs of pleopods, 

 and that those larvae with normal development which have three should be placed 

 among the Callianassidae. There remains some element of doubt, since, in the specimen 

 described as D. XIII, it is suggested that the pleopod on somite 2 may develop later 

 than the others. 



(3) Exopod on leg 5. Since Axius and Calocaris have only a vestigial exopod it is quite 

 possible that other Axiidae may have lost it altogether, and in this series of larvae there 

 is evidence that this may happen. There is, however, no evidence that any Callianassid 

 larvae of type I ever has this exopod, and its presence may still be regarded as char- 

 acteristic of Axiidae. On the other hand, among the species described in Group II C, 

 with abbreviated development, there is one which has an exopod on leg 5 and no 

 epipods, so that in this group no distinction is possible. 



(4) Telson. The statement of known larval characters shows that precisely the same 

 form of telson is found in Axius and in Callianassa type II. Many of the species dealt 

 with here show a type of telson which is strikingly different from either of the types 

 previously known. In its extreme form, as in D. I, it is long and narrow, spines 1-4 are 

 widely spaced and lateral, and the posterior margin is greatly produced and bears more 

 than the normal four spines on either side of the median spine. I have called this a 



