582 HIGHER FISHES 



process, and is lacking where the process itself is absent (as well as in 

 many fishes which do have the process) . 



The falciform (/. e., sickle-shaped) process is perhaps foreshadowed in 

 holosteans (Lepisosteus) , but it comes into prominence only in the tele- 

 osts. In its fullest development it is a ridge, formed of pigmented and 

 vascular chorioidal tissue, which projects upward into the vitreous cavity 

 from the floor of the eyeball. This protrusion of the chorioid through 

 the retina is permitted by the fact that the lips of the embryonic fissure 

 of the optic cup have never closed (see pp. 104-7). The falciform 

 process consequently runs ventrally from the optic nerve head, and also 

 veers nasally, tracing the course of the old embryonic fissure. Occasion- 

 ally it appears to commence above the optic nerve; but this 'dorsal ap- 

 pendix' of the process is always unpigmented and lies on the retina, 

 never projecting through it from behind. 



There are great variations in the form and extent of the falciform 

 process. It may be tall either proximally or (particularly, in physostomes) 

 distally, or may be low throughout its length (most physoclists) . In the 

 needlefish, Be lone, it is so tall and thin that it forms a partition in 

 the whole ventral half of the eyeball, running from fundus to iris with 

 contact on the lens over a full half-circle, and serving (through its 

 elasticity) as the quick-acting antagonist of the muscle of accommo- 

 dation in this agile fish. The process, if reduced in longitudinal extent, 

 is always present distally (i.e., toward the ora terminalis) and absent 

 proximally (toward the fundus). Never does it commence at the optic 

 disc and run only part-way to the ora; for any partial healing of the 

 embryonic fissure in the sensory retina (tending to shorten the falciform 

 process) always has a proximal-^distal direction.* Instances in which 

 the length of the process is thus somewhat reduced include the stickle- 

 backs, wrasses, blennies, some cods, and some herrings. It is present only 

 near the ora in minnows, pipefishes and sea-horses, clingfishes, the sprat, 

 and the (American) pollack. Thus, there may be variations within a 

 family (e.g., Clupeidx, Gadidae). 



Where the falciform process is wholly lacking (e.g., elephant-fishes, 

 eels, trunkfishes and puffers, anglers and batfishes), and in some in- 

 stances where it is present but only far distally, there is a system of 



* Since the definitive vitreous of vertebrates is always secreted by the sensory retina, it has 

 a slit-like defect in teleosts — the 'vitreous cleft' — above the falciform process, which locally 

 prevents such secretion (see Fig. 105a, vc; p. 261). 



