478 Mr Borradaile, Month-parts of certain Decapod Crustaceans 



A Note on the Mouth-parts of certain Decapod Crustaceans. By '• 

 L. A. Borradaile, M.A., Fellow and Tutor of Selwyn College, 

 Cambridge, and Lecturer in Zoology in the University. 



[i?ea<?16May 1921.] I 



In that close study of the relation of aquatic organisms to their 

 surroundings which is now becoming an important part of Biology, 

 much attention is given to their modes of feeding; and one of the 

 most interesting branches of the subject concerns those organisms 

 which obtain their food from organic particles, alive or dead, in 

 suspension in the water. Orton has investigated this habit in 

 various animals (Molluscs, Tunicates, etc.), and Potts has shown 

 that it is practised by the Coral-gall Crab Hapalocarcinus, and that 

 the mouth-parts of this animal are correspondingly modified, the 

 endopodite of the third maxilliped and the exopodites of the second 

 and first being provided with fringes of bristles which are used 

 for gathering the food, while the inner jaws are reduced in the 

 absence of the need for powerful organs to masticate it. These 

 adaptations are analogous to those that appear in Cirripedia and 

 Branchiopoda, which are also feeders upon suspended matter. 



Sundry other crustaceans which live in the mantle-chamber or 

 pharynx of sessile or subsessile organisms must be presumed to 

 get food of the same kind at second hand. Some time ago, in the 

 course of a study of the prawns of the subfamily Pontoniinae, of 

 which most members are commensal and a number live in the 

 mantle cavity or pharynx of bivalve molluscs and ascidians, I 

 endeavoured to discover some correspondence between the habitat 

 of the animals and the structure of their mouth-parts; but was 

 disappointed to find practically no trace of this. The jaws of the 

 crab Pinnotheres, which lives between the valves of the shell of 

 Lamellibranchs, show the same absence of specialisation in the 

 direction in which Hapalocarcinus is specialised. They have some 

 interesting peculiarities, but not those that might be expected from 

 an animal which was nourished upon finely divided food. Probably 

 these cases are to be explained by the fact that the crustaceans feed, 

 not upon food in suspension, but upon the strings of mucus in 

 which it is entangled by their host. Orton has recently shown that 

 this is done by Pinnotheres. 



A like explanation, however, cannot be given for Porcellana 

 platycheles. This animal is not a commensal, and gathers for itself 

 suspended food, taking it by means of long fringes upon the third 

 maxillipeds, but is provided with well-developed inner mouth- 



