592 ORDER A POD A. 



structed so far on the cirripedial type ; but how are we to 

 account for the extraordinary reversed position of the united 

 mandibles and maxillae, with their backs almost touching 

 each other, and their toothed edges twisted round so as to 

 face outwards in a manner unexampled, I believe, in any 

 other articulate animal ? It might, perhaps, be at first sus- 

 pected, that the compounded mandible had not really been 

 twisted round, but that the teeth had been abnormally de- 

 veloped on the outer convex margin : this view, however, 

 certainly cannot be admitted, for the properly outer convex 

 margin can be traced running far down the mouth, in a 

 manner utterly inexplicable, if this were really the inner 

 side ; and equally inexplicable on this view would be the 

 position of the ligamentous apodemes. Hence I cannot 

 doubt that this compounded mandibular organ has really ro- 

 tated on its axis ; and if the course of development could 

 be followed, I suspect that the twisting would be seen to be 

 effected as follows : we know in all cirripedes that the outer 

 and inner maxillae, and to a certain extent the mandibles, 

 instead of facing each other, are directed towards the labrum; 

 they therefore have already been twisted round a quarter of 

 a circle, as may be seen in the diagram (PL 24, fig. 4), 

 copied from the mouth of Ibla. Now let us drive inwards 

 the front of the mouth, along a narrow medial line ; these 

 organs would then (fig. 5) be compelled to turn round a 

 quarter of a circle more, and so face directly outwards. In 

 this process, the integument between the lower and outer 

 part of the mandible and the base of the palpus, which 

 normally are in close contact, would have to be greatly 

 stretched. By a movement of this order, the mandibles 

 would come to stand posteriorly or exteriorly to the other 

 gnathites; and as far as I could make out (previously to my 

 having any theory) the large single toothed portion of the 

 compound organ which most resembles a mandible, did 

 really stand outside the other toothed portion. 



With respect to the action of this singularly constructed 

 mouth ; if its ventral and oblique surface were applied to 

 any yielding object, as the adjoining soft prosoma of the 

 Alepas, the compound mandibles would be worked within an 

 absolutely closed chamber. The action of these mandibles 



