7S BALANIDiE. 



as invariably present as in the Lepadidse : these two 

 muscles are attached at their lower ends to the outer mem- 

 brane of the month, close to its basal articulation : the 

 outer one of these two muscles would, I presume, act as 

 an elevator, and the inner one as a depressor ; the free 

 part of the organ working on the top of the apodeme, like 

 an axe, on a hinge, on the top of a pole. But there is also 

 a larger depressor muscle, in an analogous position with 

 that (i. e. the first-mentioned muscle) of the mandibles ; and 

 a fourth muscle, crossing the latter depressor at nearly 

 right angles, and attached (as far as I could make out) on 

 the side of the orifice of the oesophagus, close under the 

 mandibles: the action of this latter muscle would be to 

 draw the whole organ towards the labrum. 



I must not conclude my description of the mouth, 

 without drawing particular attention to its peculiar com- 

 pounded nature. It is prominent, and is capable, as a 

 whole, of movement ; it is separated from the body by a 

 fold or articulation, which can be traced all round. It is, 

 as we have seen, composed of a broad labrum and three 

 pairs of gnathites ; which latter have only their terminal 

 segments free ; and these surround a conical hollow, at the 

 bottom of which lies the opening of the oesophagus. The 

 prominence of the whole mouth appears to result from the 

 lateral fusion of the tw T o basal segments of the three pairs 

 of gnathites. I have examined the mouth of ordinary 

 Crustaceans, and can see no trace of a structure like this. 

 That there has been some union of the parts is indisputa- 

 ble ; for the palpi, which in ordinary Crustaceans are quite 

 free, are here firmly united to the upper and outer corners 

 of the labrum ; and indeed, at first appear to be more inti- 

 mately connected with the labrum than with the mandibles. 

 The palpus on its upper and exterior surface, is in direct con- 

 tinuity with the square thickened piece of membrane, on 

 which the mandible is articulated, and likewise with that 

 side of the upper or free portion of the mandible which 

 faces the labrum. This face of the mandible, beneath the 

 toothed edge, is hollowed out or arched (PI. 26, fig. 5,/>), 

 owing to the above-mentioned continuity of its membrane 

 with that of the palpus. On the lower surface, the palpus is 



