DR. C0BE0L1) 02f THE EYE OE THE COD-EISH. 



151 



this situation. On the application of acetic acid, they lost their 

 normally plastic character, becoming uneven at the surface, solidi- 

 fied, and brittle, at the same time exhibiting a darker outline, due 

 to an increased power of refracting light ; caustic potass, on the 

 other hand, slight discoloured and dissolved them. 



General Conclusions. 



I. That the opake, white, stellate cysts which are almost always 

 present in the sclerotic of full-grown Cod-fishes contain psoro- 

 spernrige, or minute cells of a vegetable character. They may be 

 regarded as non-ciliated zoospores, and, as others have supposed, 

 may possibly have some genetic relation with the so-called pseudo- 

 naviculae of Gregarince. As a group they should be removed both 

 from the Entozoa and Protozoa, and be placed among the Chloro- 

 spores, somewhere in the immediate neighbourhood of the Palmel- 

 laceae and Desmidiacese. 



II. That the so-called choroid gland of the Cod and other 

 osseous fishes is neither glandular, muscular, nor erectile in struc- 

 ture, but is a simple form of vascular plexus, in which the capilla- 

 ries are in immediate contact with, and parallel to, one another. 



In consequence of this arrangement, this falsely so-called gland 

 is in no way connected with the adaptation of the humours of the 

 eye to varying focal lengths, but is rather to be regarded as a 

 specialized organ fitted to diffuse and modify the flow of blood in 

 a position where, from the proximity of the heart, a strong impulse 

 would interfere with the formation of a correct image on the 

 internal lining membrane of the choroid. 



The rete mirabile of Cetacea, and other similar vascular peculi- 

 arities observable in the Sloths, Lemurs, and in the long-necked 

 Herbivora, are analogous formations, concerned in the diffusion and 

 retardation of the circulating fluid for various purposes. 



In the active-moving cartilaginous fishes, where no choroid 

 gland exists, the impulse of the circulating current is lessened by 

 other anatomical arrangements, as may be seen, for example, in the 

 Porbeagle (Squalus cornubicus), where the arterial jets are broken 

 up by the alternating action of three rows of semilunar valves 

 placed within the bulbus arteriosus. 



The fleshy protuberance described by Professor Eymer Jones 

 (in his ' General Outline of the Animal Kingdom ') as occurring 

 in the arterial bulb of the Angler (Lophius piscatorius) has no 

 existence in nature. The description appears to have been bor- 

 rowed from Tarrell. 



