712 



ZOOLOGY 



SECT. 



supra-intestinal visceral connective, which bends across to the left, 

 over the oesophagus, and, some distance to the left of the alimentary 

 canal, expands into a triangular supra-intestinal visceral ganglion 

 (supra, g.), situated below the superficial layer of muscular fibres. 

 The left pleural ganglion gives off an infra-intestinal visceral con- 

 nective, which passes obliquely backwards and to the right, below 

 the alimentary canal, to a ganglion situated a little to the right 

 of the middle line the infra-intestinal visceral ganglion (infra, g.). 

 The supra-intestinal ganglion gives off a nerve which runs towards 

 the osphradium and ctenidium, which it supplies with branch 

 nerves, and unites with a stout mantle-nerve (I. mant. n.), which is 

 given off from the left pleural ganglion. The right pleural also 

 gives off a stout connecting nerve to the infra -intestinal ganglion. 

 From the supra- and infra-intestinal ganglia the left and right 



visceral connectives are continued 

 backwards and unite behind in the 

 neighbourhood of the stomach ; 

 each ends in a triangular abdo- 

 minal ganglion (I. abd. g. ; abd. r. 

 g.), and these are joined by a 

 transverse commissure, from which 

 a number of visceral nerves (vise, 

 n.) are given off. A remarkable 

 torsion of the nerve connectives is 

 here to be observed, the two 

 visceral connectives becoming 

 twisted into the form of the 

 figure 8. 



The organs of special sense 

 of Triton, in addition to the ten- 

 tacles and the osphradium, which 

 have been already referred to, are 

 the eyes and the statocysts. The eye (Fig. 628) is a rounded 

 invagination of the epidermis with an inner wall or retina (ret.} 

 composed of pigmented and sensory cells. The latter (retinophores) 

 are elongated cells narrowed at their central free ends, and pro- 

 duced at the opposite extremity to become continuous with nerve- 

 fibres of the optic nerve. The former (retinulce) have their free 

 extremities much enlarged, and surround the slender ends of the 

 retinophores. A layer of short rods (rds.) lies within the retina 

 proper. The outer wall is thin, and, with the overlying epidermis, 

 forms a transparent cornea. In the interior of the eye is a clear 

 rounded lens (I.) of dense cuticular matter secreted by the cells 

 of the retina ; this is surrounded by a less dense vitreous bod}-. 



The sexes are distinct. There is a single gonad ovary or testis 

 as the case may be lodged in the visceral spiral. The sperm - 

 duct is a white tube, thickish and much convoluted where it leaves 



FIG. 628. Triton. Section of the eye. 

 co. cornea ; ep. epidermis ; /. lens ; opt.n. 

 optic nerve ; rds. layer of rods (the line 

 is not continued far enough inwards) ; 

 ret. retina. 



