MICROSCOPE IN-DOORS. 61 



occasionally used in cleaning the apparatus, they 

 often present themselves as foreign substances. 



The hair of the bat (Fig. 181, PL 7) presents a 

 singular instance of the projection of the scales, 

 or cells, in a regular form. Hairs are not often 

 perfectly round ; in the peccary they are oval 

 (Fig. 182, PL 7); and if a transverse section of this 

 hair is examined, it will be found that the cortical 

 substance projects completely into the pulpy part 

 of the hair in several places, so as to break up the 

 pulp into several separate sections. 



In some cases it is not easy to distinguish 

 between outside and inside structure, as is seen in 

 the hair of the musk-deer (Fig. 183), in which the 

 whole is found to consist of a mass of hexagonal cel- 

 lular tissue, similar to that seen in the pith of plants. 



Insects are frequently covered with hairs, espe- 

 cially in their larva, or caterpillar state. These 

 hairs, when stiff and sharp, penetrate the skin, 

 and produce irritation there. This is the case with 

 the large tiger caterpillar. The hairs of this cater- 

 pillar are furnished with a series of barbs, which, 

 when they once penetrate the skin, are not easily 

 removed (Fig. 184, PI. 7). 



Spiders are frequently covered with hairs, some 

 of which are branched (Fig. 185, a), others present 

 a spiral appearance (b), whilst, again, others offer a 

 series of small bristle-like hairs running down each 

 side of the primitive hair (c). 



Many of the Crustacea have hairs upon their 

 shells. Those upon the nabelluni of the common 

 crab have minute bristles on one side of the 

 parent stalk (Fig. 186, PI. 7), so as to form a little 

 comb, with which to brush off the impurities from 

 its branchiae. A live crab from the aquarium may 

 be watched for the purpose of observing these 

 cleanly movements. 



