46 Papers from the Department of Marine Biology. 



Caiman says: 



"The buccal area is very large, extending across the whole width of the 

 carapace in front. Its anterior margin is sinuous, curving forwards on either 

 side below the eye and taking the place usually occupied by the inferior 

 margin of the orbit. The median part of the buccal margin approaches so 

 closely to the base of the antennules that an epistome can hardly be said to 

 exist. The third maxillipeds do not nearly cover the buccal cavity, and are 

 widely separated from each other at the base by a semicircular area of the 

 sternum. The ischium is flattened, subtriangular in shape, widening gradu- 

 ally from a narrow base, and having its antero-internal angle produced for- 

 wards, rounded, and fringed with setse. The merus is articulated at the outer 

 end of the distal margin of the ischium, and is less than half the width of the 

 latter, hardly wider than the succeeding joints, and but little flattened. The 

 exopod is rudimentary, being a simple lobe about half the length of the 

 ischium. The epipod is well-developed. The second maxillipeds have the 

 basal part of the exopod much expanded. In the first maxillipeds, also, the 

 same part is very stout and much stronger than is usual in this appendage, 

 while the inner lobe or endopod is small and subtriangular." 



But no mention is made here or elsewhere of the maxillae and mandi- 

 bles. Caiman had so little material that he was unable to investigate 

 this point. But in the related genus Cryptochirus Heller had discovered 

 and figured these appendages so long ago as 1861. They are of a very 

 remarkable type, and as this is practically identical in the two genera I 

 quote the passage in which he describes them: 



"Die beiden Maxillen sind ebenfalls blattchenformig, ebenso die nach 

 innen stark verbreiterten und mit geradem scharfen Kaurande versehenen 

 Mandibel. Der Stiel der letzteren bildet mit der kauplatte einen starken 

 Winkel, ein Palp fehlt." 



When I came to examine Hapalocarcinus, not knowing the above 

 passage, I was at once struck by the fact that each member of the two 

 pairs of maxillae is reduced to a single elongated plate while the mandi- 

 ble is without a palp and has not the robust biting character of the 

 usual Decapod mandible. 



Before describing these organs in detail I will state what, in my 

 opinion, is the reason for their anomalous condition. So striking is 

 their divergence from the very uniform Brachyuran type that a definite 

 explanation is to be expected. This is without doubt to be found in a 

 change in the feeding habits of the animal during its recent evolution 

 caused by its voluntary imprisonment in an almost totally closed space. 

 The usual diet of crabs, the comparatively large fragments of animal 

 or vegetable substance, is denied to Hapalocarcinus. There is no 

 doubt, indeed, that this form must live on the plankton which is drawn 

 into the gall with the respiratory current, and since in the "closed" 

 gall the holes which allow entrance to the interior are exceedingly small 

 in the skeleton and must be smaller still if we allow for the coating of 

 living co3nosarc, it is evident that the larger constituents of the plank- 

 ton will be unable to pass through them. Hapalocarcinus is thus an 



