48 



Papers from the Department of Marine Biology. 



and help to rectify the inner border and internally the appendage is 

 thickly provided with short hairs, so that though the appendages do 

 not meet exactly in the middle the effectiveness of the screen is thus 

 completed. 



Pinnotheres is a genus of crabs in which a resemblance to Hapalo- 

 carcinus might be expected, for its members live in similarly secluded 

 positions, either the mantle cavity of molluscs or the tubes of worms; 

 and both types of host being plankton feeders, they might be expected 

 to share the diet. This does not appear to be the case and the oral 

 appendages are very different. But the third maxilliped departs very 

 considerably from the normal type. This is due to the fact that the 

 abdomen has increased greatly in size, so that the buccal cavity has 

 been correspondingly restricted, diminishing very greatly in depth. 

 As a consequence the third maxillipeds are placed in a very different 

 position, running almost at right angles to that which they occupy in 



D 



FIG. 6. Various types of third maxilliped in the Brachyura. 



A. Cancer, XI. B. Hapalocarcinus, X100. C. Stenorhynchus, X4. D. Porcellana, X6. 

 ex., exopodite; i, ischiopodite; m, meropodite; c, carpopodite; p, propodite; d, dactylopodite. 



other crabs. But they still form a double trapdoor, a little incomplete 

 on the posterior border, where the deficiency is made up by a thick 

 fringe of setae. In the fusion of three joints of the endopodite of the 

 third maxilla, Pinnotheres appears to differ from all other Brachyura. 



In Ranina among the Anomura a structure and disposition of the 

 third maxillipeds are found similar to that occurring in Stenorhynchus. 

 The basal articles of the endopodite are not enlarged, but the three 

 last joints are bent down to fill in the gap and all are strongly setose. 

 Curiously enough, when Milne Edwards described his geimsLithoscaptes 

 (which is generally acknowledged to be synonymous with Hapalo- 

 carcinus) he compared the third maxilliped with that of Ranina. The 

 only resemblance lies in the slender meropodite and the setose borders. 



In Hapalocarcinus the arrangement of the buccal area is quite 

 unique. It is very wide, so that the third maxillipeds are set far apart. 

 The ischiopodite of the endopodite is rather broad, but the meropodite 



