304 ADAPTATIONS TO SPACE AND MOTION 



of the voluntary eye movements. A list of species known to have a fovea, 

 compiled chiefly from the recent work of Kahmann, follows : 



With good fovea : 



Girella sp. Julis geofredi 



Hippocampus spp. Blennius basiliscus 



Siphonostoma typhle Blennius gattorugine 



Syngnathus acus Blennius sanguinolentis 



Syngnathus tenuirostris Blennius tentacularis 



Serranus cabrilla Blennius ocellaris 



Serranus hepatus Blennius pavo 



Serranus scriba Pholis gunellus 



With fovea, or at least the beginnings of one : 



Balistes capriscus Trachinus vipera 



Balistapus aculeatus Julis vulgaris 



Tetrodon fluviatilis Julis pavo 



Trachinus draco Agonus cataphractus (?) 



These foveate fishes are all marine and inhabit the littoral zone. Some 

 are characteristically agile and lively in the pursuit of prey, though others 

 are sluggish, and the pipe-fishes and sea-horses have very deliberate swim- 

 ming habits. Several of the species inhabit rocky clefts, where their 

 capacity for eye movements seems a definite advantage in their cramped 

 quarters. Rather surprisingly, considering its behavior, the archer-fish 

 {Toxotes jaculator) is not among those which have a fovea; and it will be 

 recalled (p. 292) that this fish also has a rather narrow binocular field. 



In all of the above species except the sea-horses, the fovea is located 

 strongly temporally, in the retinal region which can see binocularly (Fig. 

 77b, p. 185). But while these fishes can and do converge their eyes to 

 aim both foveae at a prey object, the eyes are moved independently and 

 are not conjugated, but only coordinated temporarily in each act of 

 convergence. In fact, such fishes are the only vertebrates which can em- 

 ploy a temporal fovea for monocular vision. In such genera as Blennius, 

 Serranus, Julis, and Trachinus, either monocular or binocular fixation 

 may be maintained on an object. The better the fovea, the greater the 

 tendency to adhere to binocular fixation. The average teleostean fovea 

 is a shallow pit, far inferior in construction to sauropsidan foveae; but 

 in Girella, according to Mile. Verrier, it is the equal of the superb fovea 

 of the chameleons. Some syngnathids have been claimed to have two 

 foveas in each eye, but Kahmann was unable to confirm this. 



Many other species, among those kept in large American aquariums, 

 can be seen to make spontaneous fixative movements. Most of these have 



