272 ANIMAL BIOLOGY. [Part II. 



occupies a considerable space on each side of the head. There 

 are about 1800 facets on each eye. These facets are derived 

 from the cuticle ; they are hexagonal (as a rule) in outline, and 

 biconvex in cross-section. Beneath each facet is a short stout 

 crystalline cone coated with dark pigment. The apex or deeper 

 part of the cone is imbedded in a so-called nerve-rod or rhabdom, 

 composed of four elements (rhabdomeres), which diverge above to 

 receive the point of the cone. Around the rhabdom is a proto- 

 plasmic sheath, inconspicuously divided into four segments or 

 retinulce, which have been found to contain " visual purple," 

 such as is found in the vertebrate retina. Beneath this re- 

 tinula layer is a basilar membrane perforated by a number of fine 

 holes. The retinulae and rhabdoms constitute the ultimate nerve- 

 endings, and are connected through the fenestrations of the 

 basilar membrane with nerve-fibrils. The basilar membrane is 

 homologous with the basement membrane of the cuticle; all 

 between this and the facets is probably hypodermic in origin. 



The optic nerve given off from the cerebral hemisphere of the 

 brain gives rise, in the cockroach, before it reaches the basilar 

 membrane, to two ganglionic enlargements. In the fly and bee 

 there are three such ganglionic swellings. Each ganglion in the 

 cockroach (and the inner two in the bee) is formed of a network 

 of fine fibrils, surrounded by a cellular sheath of densely-packed 

 nerve-cells, with large nuclei surrounded by a delicate proto- 

 plasmic envelope. The fibres connecting the two ganglia decus- 

 sate or cross ; and the fibres between the outer ganglion and the 

 basilar membrane do so partially. 



It has recently been urged that not only the layer of retinulse 

 and rhabdoms, but the inner and outer ganglion as well, should 

 be regarded as constituting the insect retina ; in which case the 

 retinulse and rhabdoms answer to the layer of rods and cones in 

 the vertebrate retina. The order of parts in the optical appar- 

 atus of the cockroach is therefore : Brain, optic nerve, retina, 

 dioptric apparatus. Of these the retina is constituted by inner 

 ganglion, decussating fibres, outer ganglion, partially decussating fibres, 

 retinula layer. While the dioptric apparatus may be represented 

 by corneal facets and crystalline cones. 



