Hapalocarcinus, the Gall-forming Crab, etc. 55 



forming crab. But on examination I found its stomach to be full of 

 fragments, organic and inorganic, which showed a varied diet, and whose 

 possible source was rather a puzzle to me. The walls of the stomach, 

 moreover, showed a typical armature well fitted for mastication. The 

 buccal appendages, though peculiar in some respects, showed little 

 evidence of reduction in any way comparable to that in Hapalocarcinus. 



Similarly, I examined the stomach of Synalpheus brucei, which I 

 describe in another paper as a commensal with crinoids on the Pacific 

 coral reefs, living for the most part if not entirely within the circle of 

 their arms. It was in all cases crammed full of foraminifera, radiolaria, 

 spicules of calcareous and siliceous sponges, and crustacean fragments, 

 as well as pieces of coral skeleton. This examination showed how wide 

 is the dietary of these quasi-sedentary creatures. 



While no other member of the Decapoda shows similar modifications 

 of the alimentary apparatus, there is a curious similarity between the 

 oral appendages of Hapalocarcinus and those of an entire division of 

 the lower Crustacea, the Branchiopoda. The following quotation 

 from Sedgwick's Textbook of Zoology (Article Crustacea, by J. J. 

 Lister, p. 369) summarises the conditions prevailing in the latter group : 



"It is remarkable that while the thoracic appendages of the Branchiopods 

 conform so uniformly to a common and, as it appears, primitive type of struc- 

 ture, the appendages about the mouth present the greatest divergence from 

 that type which is met with in any group of Crustacea. The absence of the 

 mandibular palp in the adult is all the more striking because of its size and 

 frequently biramous character in other Entomostracan groups, the Copepods 

 and the Ostracods. Again the two pairs of maxillae, which even in the Mala- 

 costraca conform more closely than any other appendage to the primitive 

 'phyllopod' type of limb, are here, in the Branchiopods themselves, perfectly 

 simple lobes, and one pair is often missing. In adult Cladocera and in Lim- 

 netis among the Conchostraca there is only one pair; in Branchipus the second 

 pair is rudimentary, and in this genus, as also in Apus, where two pairs are 

 present, they are retarded in development, appearing later than the thoracic 

 appendages which follow them in position. . . . The late appearance of 

 the maxillae is perhaps a confirmation of the view taken here that the simple 

 condition of those appendages in the Branchiopods is a specialised and not 

 a primitive character. The small size and simple character of the maxillae 

 is perhaps dependent on the peculiar habit of members of this genus ... of 

 passing the food forwards along the ventral groove." 



The absence of the mandibular palp and the reduction of both pairs of 

 maxillae to a simple lobe are thus characters common to both the crab 

 and the tiny Entomostracans. Lister concludes that this condition 

 is secondary in the Branchiopoda and a similar conclusion with regard 

 to Hapalocarcinus can not be avoided. Without doubt, also, the 

 reduction in both cases is associated with particular habits of food 

 collection, but the explanation in the last paragraph of the quotation 

 is hardly adequate. A few remarks on the method of feeding in the 

 Branchiopoda may be made before instituting a comparison with 



