Spaulding, Association in Hermit Crabs. 53 



tory. " The gustatory hairs, lie as two patches of "minute curi- 

 ously flat organs" on the under surface of the outer filament of 

 the antennules, the innermost appendages, which observation 

 shows are kept moving constantly. There are no sense organs 

 in the mouth. ^ 



At the basis of each antennule is a little sac formed by an 

 infolding of the chitinous integument, communicating freely 

 with the water, and containing little sand grains or otoliths. 

 Here are present a second kind of sensory hairs, connected 

 with the central nervous system by branches of the antennulary 

 nerve, and whose function is either that of audition or of equi- 

 librium.^ The third class of hairs are tactile in function, and 

 are especially numerous on the antennae, i. e., the second pair 

 of appendages, although there are some on the antennules, the 

 other appendages, and the remaining integument. 



The two eyes of the crab are compound or facetted and are 

 seated on movable pedestals. They are covered by a transparent 

 chitinous cuticle, forming a cornea ; this is divided into facets, be- 

 neath each of which there is an ommatidium with two segments 

 (a) an outer, which is vitreous and refractive, and an inner, a 

 short retinula, which is sensitive, thus giving a structure analo- 

 gous to rods and cones. These cones are surrounded and so 

 separated from each other by a pigment ; their apex is em- 

 braced by elongated cells in the midst of which is a fibril of the 

 optic nerve. Each facet functions as a single eye and therefore 

 like the vertebrate eye gives no sensitive continuum but, rather, 

 "mosaic vision," i. e., various images in juxtaposition. The 

 eye as a whole is supposed to give a vision of distinct objects 

 and space relations. 



The brain is formed from the first three pairs of embryonic 

 ganglia, and is therefore a "syn-cerebrum"; it supplies the eyes, 

 the antennules and the antennae with nerves. It is connected 



1 VOM Rath, Otto. Zur Kenntniss der Hautsinnesorgane der Crustaceen, 

 Zoologischer Anzeiger, 365, 1 89 1. 



■■' Hensen (Studien iiber das Gehororgan, Zeitsch. f. wiss. Zoolos^., Bd. 13, 

 1863) says they are auditory, while Delage, Archiv. d. Zool. Exper., 1887, (2), 

 T. 5, says they are for position and qeuilibrium. { 'ited by voM Rath.) 



