FISHES 



299 



be partially filled by a similar accessory body, the lentiform body. 

 It occurs in the majority of Teleosteans (Erdl, 1839) — according to 

 J. Miiller (1840), in all those provided with the hyoid gill (or pseudo- 

 branch) from which it is directly supj^lied with highly oxygenated 

 arterial blood ; from the "gland" the blood flows into the choroidal 

 circulation. Both the pseudobranch and the choroidal gland are 

 absent in some genera with small eyes, such as the eels [AngnUla) and 

 the cat -fishes (Siluroids). 



It has been suggested that the choroidal gland forms a special mechanism 

 whereby the circulation is maintained despite considerable changes in pressure 

 when rapid alterations occur in the dejith of swimming (Allen, 1949) ; this, 

 however, seems unlikely in view of its constant presence whatever the habitat 

 of the fish. Nor does it appear to act as an erectile organ assisting accommodation 

 by pushing the retina forwards (Barnett, 1951 ; Yamasaki, 1954) ; it is probably 

 pvirely nutritive in function. 



Anguilla 



Cat-fish 



The vascularisation of the inner eye is further maintained by the 

 falciform process, or when it is absent, by a hyaloid system of vessels. 

 The FALCIFORM PROCESS is a peculiarity of Teleosteans and consists of 

 a prominent sickle-shaped ridge of pigmented and richly vascularized 

 choroidal tissue which j^rotrudes through the inferior part of the retina 

 in the region of the foetal fissure (which has never closed), running from 

 the optic disc to the ciliary region (Figs. 330-1) (Franz, 1910). This 

 structure is somewhat analogous to the cone of Reptiles and the pecten 

 of Birds although these structures are ectodermal in origin and are 

 secondarily vascularized. In some species the fissure has closed 

 posteriorly so that only the anterior portion of the falciform process 

 remains (the cod. Gadns : herring. Clujiea : carp. Cyprinus : etc.). In 

 those species in which the process is small or absent, as in certain eels 

 (conger. H. Virchow, 1882), cyprinoids such as the carp and roach 

 (O.Schultze. 1892), and goby fishes (Karsten. 1923), the nutriment of the 

 inner eye is taken over by a hyaloid system of vessels which, like the 

 falciform process, issues through the foetal fissure : the main artery 

 enters the eye in the region of the oj^tic disc and instead of running 

 through the choroid to constitute the basis of the falciform process, 

 breaks into the superficial layers of the vitreous and forms a dense 

 vascular plexus running anteriorly lying loosely upon the inner surface 

 of the retina (Chrustschoff, 1926) (Figs. 332-3). This membrana 

 VASCULOSA retix.e Constitutes an arrangement of widespread dis- 

 tribution among Vertebrates and is comparable to that seen in certain 

 Amphibians and Reptiles (snakesj. It is to be noted that these vessels 

 ramify in the vitreous, lying superficially on the retina without 

 entering it. The veins drain anteriorly into an annular vein which 

 leaves the eye through the ciliary zone, and between tlie two a widely- 



Gadus 



Clupea harenytxs 



