166 
MARINE FAUNA OF ST. ANDREWS. 
The great development of this organ, its central calcareous 
bow, and long hairs are thus explained. The branchial lamina} 
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 flat 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 the branchial laminse. The fifth pair as a whole would 
seem to be connected with the buccal rather than the 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 
the mouth above the chitinous tissue filling up the posterior 
notch of the mandible. The tuft of long hairs (e, loc. cit.), 
however, may render some assistance to the branchial portion 
of the fourth pair of foot-jaws in contact with it. 
The appendage of the mandible (a, fig. 7, op. cit.) seems to 
have a considerable influence in the prehension and direction 
of the food between the maxillae ; 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 horizontal, 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 u commensalisme ” 
of Prof, van Beneden. Nemertes carcinophila abounds on 
the hairs bearing ova; and the young of the edible mussel 
and other adventitious growths are common, besides Sacculina 
(Plate IX. fig. 13), and Trematode larvae 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). 
