HARDWICKE'S SCIENCE-GOSSIP. 



129 



is here again taken of the animal's transparency ; some 

 of the lenses are seen to look directly backwards 

 through the substance of the body. One of these 

 posterior lenses is seen in the drawing to be very well 

 developed. 



At 0. g. is seen the optic ganglion, the ultimate 

 structure of which was not visible. From this to the 

 back of the eye are seen running a number of delicate 

 nerve-fibres ; these can be traced into the pigment, 

 but the way in which they end cannot be seen, owing 

 to the opacity of the pigment. 



The movements of the eye are effected by four 

 muscles, corresponding in position and action with the 

 four recti muscles of the human eye. 

 They may be termed the superior, 

 inferior, right, and left muscles of 

 the eye. They are each composed 

 of one or two unstriped muscle cells, 

 with well-marked nuclei. In some 

 specimens the muscle is branched 

 at the insertion into the eye. They 

 arise from a common origin at the 

 back of the optic ganglion. In the 

 illustration, the letters i.r. point to 

 the inferior muscle. 



The movements of the eye during 

 life are incessant, and the action of 

 the muscles can be readily made out. 



Whether the movements are 

 voluntary or not, cannot be ascer- 

 tained, but from the structure of the 

 muscular fibre, it may be presumed 

 that they are involuntary, since the 

 animal possesses the striped variety 

 in other parts of the body. 



James Harvey. 



inverted groined arch, the centre of which is pierced 

 by, and supports, a long seta. Externally, each areola 

 is protected by a transparent chitinous dome, per- 

 forated at its summit with an aperture sufficiently 

 large to permit free movement in every direction of 

 the long seta passing through it, as seen in Fig. 65. 

 From specially prepared sections of the pygidium, 

 nerves are seen whose ends expand into large globular 

 masses, each surrounding the base of an areola, and 



Fig. 64. — Disk-like areola, 

 front view, much enlarged. 



65. — a, areola ; c, chitinous cutic'e ; d, dome, 

 side view, much enlarged. 





THE PYGIDIUM OF THE 

 FLEA. 



THE pygidium is somewhat kid- 

 ney-shaped, and is firmly ad- 

 herent to the eighth segment of the 

 abdomen, which, by strong muscles, 

 can be drawn beneath the seventh. 

 Being bi-lobed, it forms an arch 

 above the posterior portion of the large rectum, 

 extending from the last pair of spiracles to the anus. 

 In Pulex irrita/is, each lobe is perforated by fourteen 

 cup-like bodies, or areolce ; these, when seen from the 

 exterior, under a tolerably high magnifying power, 

 have the appearance of disks ornamented with a ring 

 of rectangular rays, as represented in Fig. 64. The 

 number of rays vary from eight to eleven, according 

 to the position of the disk. From a transverse section 

 of the pygidium, the areolae are seen to extend below 

 the chitinous cuticle, the base of each resembling an 



ns 



Fig. 66. — Oblique section of Pygidium. s, long seta; a, areola ; n, 

 ns, nerve-swelling. X 210. 



enclosing the lower end of the long seta. In some 

 of these swellings a large nucleus is visible, and all 

 appear granular. Fig. 66 is taken from an oblique 

 section of the pygidium, in which nerves and their 

 swellings are seen connected with four areolae, and 

 portions of others which supply areola? not visible in 

 this section. 



Villaines has observed ganglia in the skin of insects, 

 which give off process to certain hairs ; also Kunckel 

 and Gazagnaire have discovered nerve-swellings con- 

 necting nerves with specialised hairs in the skin of the 



