?50 200LOGY 



supported by curved plates of cartilage forming a sort of orbit, 

 connected with the cranial cartilage. The significance of the 

 various parts of the eye will not be fully understood till the struc- 

 ture of that of the Vertebrata has been studied. A transparent 

 portion of the integument covering the exposed face of the eye is 

 termed the false cornea (corn). The eyeball has a firm wall, or 

 sclerotic, strengthened by plates of cartilage (scl. cart). Externally, 

 i.e., on the side turned towards the surface of the head, this presents 

 a large opening the pupil. The part of the sclerotic which imme- 

 diately bounds the pupil is termed the iris (ir.) ; it contains muscular 

 fibres by whose action the size of the pupil can, to a limited extent, 

 be increased or diminished. Just internal to the iris and projecting 

 slightly through the pupil is the lens a dense glassy-looking 

 body of a spherical shape. The lens consists of two plano- 

 convex lenses in close apposition ; it is supported by an annular 

 process the ciliary process (cil. proc.) projecting inwards from the 

 sclerotic. Between the two parts of the lens lies a thin layer of 

 cells the cornea. The lens with the ciliary process divides the 

 cavity of the eye into two portions, a smaller outer the cavity of 

 the aqueous humour, containing water, and a larger inner, con- 

 taining a gelatinous substance the vitreous humour. Over the wall 

 of this inner chamber extends the retina (ret), the sensitive part of 

 the eye, in which the optic nerve-fibres derived from the optic 

 ganglion terminate. The retina is of somewhat 

 complicated structure, mainly composed of a layer 

 of close-set parallel rods (rds), which immediately 

 (I bound the cavity of the eye, with externally a 



/j layer of retinal cells which are in communication 



sr -, with the rods internally and with the optic nerve- 



\ fibres externally. 

 f^ \ In immediate contact with the eye, in addition 



FIG 678 - Sepia tO the P tic g^g^On, is a lar g e soft bod 7 of 



cuitrata. stato- unknown function, the so-called optic gland or 

 nth, highly magni- white ^ Bundles of muscu i ar fibres bring 



about limited movements of the eyeball in various 

 directions. An integumentary fold of the character of an eyelid is 

 capable of being drawn to some extent over the false cornea. 



The statocyst (" otocyst ") (Fig. 665), though not of such compli- 

 cated structure as the eye, is very much more highly developed 

 than that of the Pelecypoda or Gastropoda. The two statocysts are 

 embedded in the cartilage of the posterior portion of the cranium 

 close to the pleuro-visceral ganglion. The cavities of the two 

 organs, which are about 3 mm. in diameter, are separated by a 

 median cartilaginous septum. The inner surface presents a 

 number of rounded and pear-shaped elevations, and is lined with a 

 flattened epithelium raised up on the posterior surface into a 

 ridge or crista acustica and a macula acustica composed of large 



