THE MOUTH-PARTS OF THE SHORE CRAB. 115 
On the Mouth-parts of the Shore Crab. By L. A. Boreadaile, Sc.D. 
Fellow and Tutor o£ Selwyn College, Cambridge, and Lecturer in 
Zoology in the University. (Communicated by Professor E. S. 
Goodrich, M.A., F.R.S.) 
(Plates 10, 11.) 
[Eead 6th April, 1922.] 
I. 
The organs which stand about the mouth of a Decapod Crustacean make up . 
a complex as intricate as any that is to be found in the Animal kingdom. 
They are also extremely important to their possessor, for without them the 
animal can neither feed nor breathe, and to one of them falls, at least in 
many cases, the duty of keeping clean the indispensable organs of special 
sense. Yet they are at preseut but little understood. 
In a study upon the Common Prawn, published in 1917(5), I endeavoured 
to solve the problem which the morphology of these organs presents, and 
made a beginning with the investigation of their working. This paper 
contains an account of some observations upon Carcinus mcenas, a species at 
the other end of the decapod series. 
The term " mouth-parts " denotes, in the Crab, a number of organs which 
stand upon the under side of the body, in the region which is bounded behind 
by the anterior edge of the mass of fused postoral sterna, at the sides by the 
edge of the inturned carapace where this encloses the exhalent passage of 
the gill-chamber, and in front by the fused antennal and mandibular sterna 
(epistomo). The sterna of the maxiilulary to second maxillipedal segments 
inclusive are more intimately united with one another than those of the leos; 
and they form, a mass, roughly triangular with the apex forwards, that 
stands out steeply from an area, in front of and beside it, which is covered 
by a thin cuticle supported upon the pieces of the endophragmal skeleton. 
It is upon this area that the mouth-parts are inserted. They are : the six 
pairs of limbs from the mandibles to the third maxiilipeds inclusive, the 
upper lip or labrum, the lower \ip or metastoma, and the fleshy opening of 
the mouth itself. I propose to describe in succession each of these parts and 
its movements, and then to discuss the functions of the complex as a whole. 
II. 
, 1. The Third Maxilliped is built upon the plan of the legs. This statement, 
by a phenomenon often seen in serially homologous structures, is true even of 
features— such as the fusion of the basis and ischium and the nature of the 
articulation of the joints — which cannot be supposed to have existed in the 
schizopod ancestor whose thoracic limbs were not differentiated from one 
