The Internal Anatomy of Spiders 



this reflection is termed a tapetum (ta-pe'tum). The supposed 

 function of a tapetum is to increase the effect of a faint light on 

 the visual organs, the light being caused to pass through the 

 retina a second time when it is reflected from the tapetum, which 

 is behind the retina. 



The structure of the tapetum varies greatly in different 

 animals; in the cat and other carnivores it is a thick layer of wavy 

 fibrous tissue; in insects it is a mass of fine tracheae; in the Crus- 

 tacea it is formed of a light-coloured pigment in the accessory 

 pigment cells of the eye; and in spiders it consists of a layer of 

 cells behind the retina containing small crystals that reflect the 

 light. 



The form of the tapetum layer varies in the eyes of different 

 spiders; three types have been described. In the first type it 

 lies immediately within the eye-capsule and encloses the whole 

 of the retina. As the inner end of the eye-bulb is more or less 

 compressed, the tapetum, as Mark has aptly described it in 

 Agelena, is a canoe-shaped structure, with a fissure along the keel, 

 through which the nerves pass to the retina. This type is termed 

 the funnel-form type. 



The second-type of tapetum is also canoe-shaped, with a slit 

 at the bottom, and is also termed funnel-form; but it differs from 

 the first type in that the funnel is more shallow, enclosing for the 

 most part, only the proximal rod-bearing ends of the visual cells; 

 and it does not lie immediately within the eye-capsule, owing to 

 the fact that the nuclear ends of many of the visual cells are bent 

 back under the sides of the tapetum cup and consequently lie 

 between it and the eye-capsule. This form of a tapetum has been 

 figured by Widman, whose figures I copy. Figure 169 1, repre- 

 sents a section of a prebacillar eye of Amaurobius cut at right 

 angles to the longer axis of the tapetum; and (Fig. 169, 2,) repre- 

 sents a section lengthwise of the tapetum slit. 



These two figures show a peculiar condition of the corneal 

 hypodermis which exists in some eyes of spiders. The nuclear 

 ends of the cells are crowded to one side; the result is that in one 

 section these cells are cut lengthwise; in the other, transversely. 



The third type of tapetum is in the form of a shallow cup 

 whose sides are divided into many parallel strips by a series of 

 cross slits running out each way at right angles to the long slit 

 at the bottom of the cup; this type is termed the grate-form type. 



164 



