166 MARINE FAUNA OF ST. ANDREWS. 



The great development of this organ, its central calcareous 

 bow, and long hairs are thus explained. The branchial laminas 

 are ranged with their edges to the afferent current, which 

 crosses the organs at right angles to their long axes, and 

 so impinges between the plates. The action of the broad 

 shield of the fourth pair of foot-jaws, again, affects the ingoing 

 stream, and plays upon the large fiat surface at the base of 

 the flabellum of the third pair. It would thus tend to spread 

 out the long hairs of the latter, and direct the current upwards 

 over tlie branchial lamina;. The fiftli pair as a whole would 

 seem to be connected with tiie buccal rather than tlie respira- 

 tory apparatus ; for the curiously twisted portion (c, fig. 6, Trans. 

 Linn. Soc. vol. xxiv. p. 88) is nicely adapted to the deep an- 

 terior notch of the mandible, while the curved portion (a) enters 

 tlie mouth above the chitinous tissue filling up the posterior 

 notch of the mandible. The tuft of long hairs (e, loc. cif.), 

 however, may render some assistance to the branchial portion 

 of the fourth pair of foot-jaws in contact Avitli it. 



The appendage of the mandible (a, fig. 7, op. cit.) seems to 

 have a considerable influence in the prehension and direction 

 of tlie food between the maxilla; ; it has lateral motion as 

 well as flexion and extension. The flexible process filling up 

 the gap in the underpart of the maxilla, and connected with 

 the lip beneath the latter, would seem to prevent the escape of 

 particles in biting and deglutition. It is attached to a firm 

 horny basis, which has free liorizoutal, but little or no vertical 

 motion, except when greatly extended. 



In females bearing eggs the muscles on the external or 

 under surface of the intestinal tract greatly increase in size at 

 the junction of the abdomen with the cephalothorax. In 

 males and females without ova the exterior of the gut is 

 sparingly supplied with such tissue. 



This crab affords a good example of the " commensalisme " 

 ()f Prof, van Beneden. Nemertes carcinopJiila abounds on 

 the hairs bearing ova ; and the young of the edible mussel 

 and other adventitious growtlis are common, besides Sacculina 

 (Plate IX. fig. 13), and Trematode larva; in the liver and other 

 parts. Various abnormalities from injury also occur. The 

 colours of the males are often remarkably bright, both on the 

 upper and under surfaces of the carapace (Plate V. fig. 8). 



