168 Proceedings of the Royal Physical Society. 



secondary bristles fewer and smaller; on the ventral sur- 

 face they are very long and slender, and there are no 

 bristles on the proximal half of the stem. Those hairs on 

 the carapace which fringe the orbit are of the same nature. 

 In this situation they function evidently as a sifting appa- 

 ratus, to prevent the entrance of any large foreign bodies, 

 especially where they overhang the bases of the thoracic 

 appendages and fill up the slight interval between these and 

 the margin of the branchiostegite ; and, while they admit of 

 the free ingress of water to the branchial chamber, they bar 

 the way to any organisms or inorganic particles that are not 

 of microscopic size. 



Experimental. — The carapace is impervious to tactile im- 

 pressions, as it is modified for protective purposes, and the 

 antennae take the place of tactile sette. 



Dorsal surface of Abdominal Somites. — The dorsal surface 



of the first two somites, which are still within the reach of 



the antennae, are almost entirely free from tactile setae ; but 



each succeeding segment has a greater number of groups of 



tactile setae, precisely similar to those in the great claw, but 



as a rule, with fewer setae in a group — averaging from six to 



twelve. They are directed backwards, and lie at an acute 



angle with the surface of the cuticle ; they are most numerous 



on the surface of the telson. There are no tubercles on these 



surfaces, and the groups arise from the bottom of slight 



depressions in the cuticle. One seta about the middle of 



the group is usually longer than the others, which converge 



to it (of course setae are entirely absent from the surfaces of 



the pleural and tergal articular facets). On the surfaces of 



the pleura there are numerous groups, and these are present 



from the 2d somite onwards, though on the 2d there are 



very few, and these close to the free edge. Here they are 



directed downwards. On the swimmerets of the last (6th) 



somite the arrangement is the same as on the telson. At the 



free edges of the pleura, except on the 1st, the disposition is 



somewhat curious. Several rows of strong fringing setae 



arise beneath the edge of the pleura, just as at the margin of 



the cephalo-thorax, and there is no admixture of tactile setae ; 



but at the apex of the angle formed by the anterior and 



