40 



and relations to the branchial pyramids. Thus whilst in the lobster 

 and the nearly allied genera, these appendages, to the number of five 

 on either side, belong to the four first pairs of ambulatory extremities 

 and to the third of the maxillary pairs, and run from below upwards 

 between the branchial fasciculi, we only find three pairs in the 

 Brachyura, belonging exclusively to the maxillary extremities, and 

 penetrating into the branchial cavities horizontally, two on the outer 

 surface of the branchia and one between the inner surface of these 

 organs and the flancs." 



I entirely coincide with Milne Edwards, that the use of the flabella 

 is not merely to ensure the formation of currents in the water, for if 

 we examine their situation in the common crab, we find that their 

 movements are not in the direction of the inlet or outlet of the bran- 

 chial cavity, but at right angles to it : they evidently answer another 

 purpose ; and now let us see what evidence the microscope will unfold 

 of their probable function. 



Having had occasion some time since to examine the hairs found 

 on different parts of the more common Crustacea, I at length came 

 to those situated on the flabella, and I observed that these hairs were 

 remarkable for having a number of large hooks developed on one side 

 only. The hairs themselves were not straight, but curved, or scimi- 

 tar-shaped, and the hooks generally occurred on the convex side. In 

 some specimens there were two rows of hooks, as seen in Plate iii. 

 figs. 12 — 19. A transverse section of No. 19 is seen at fig. 20 : this 

 specimen was taken from the common crab, Cancer Pagurus ; the hair 

 of the flabella of the small edible crab, Cancer Mcenas, is represented 

 by fig. 15 ; the other figures represent the hairs from the flabella of 

 small species of crabs from different parts of the globe, but all exhi- 

 biting peculiar modifications of one and the same plan. Many of 

 these hairs have still smaller hair-like projections on them, besides 

 the hooks, and in most cases these little hairs are inclined towards 

 the point of the large hair, and give to it an appearance of a serrated 

 edge, fig. 14. By way of contrasting these hairs with those found on 

 other parts of the same animal, I have represented the ordinary hair 

 of a crustacean of the highest class at fig. 21. 



Finding the appearance of hooks to be constant in all the species 

 examined, I at once began to speculate on their probable use, and in 

 the course of investigation my attention was directed to the structure 

 of the branchia, and on a careful examination I found that on each 

 branchial lamella there was a spine developed, which was very evident 

 even to the unassisted eye, and as there are two series of lamellae, 



