558 CYCLOSTOMES 



Strands are present even in brook lampreys, in most of which the thick- 

 ening is practically non-existent (except superiorly), and also in other 

 parasitic lampreys — none of which has it so prominent as Lampetra. 



The chorioid appears to differ markedly between parasitic lampreys 

 and the various brook species in the same genera with them. In Lampetra 

 fluviatilis and Petromyzon marinus, perhaps also in Entosphenus trident- 

 dtus (where an especially intense pigmentation interferes with obser- 

 vation), the outer half or more of the thick chorioid consists of a con- 

 tinuous lake of blood, the 'subscleral sinus'. This is presumably fed 

 directly by the choriocapillaris, which in turn is supplied by small arteries 

 in the more ordinary, inner, portion of the chorioid. The chorioid has no 

 true veins; the arteries branch away from four main ones, one in each 

 quadrant, which stem from a single artery which perforates the sclera 

 just beneath the optic nerve. The chorioidal sinus is drained through the 

 sclera by four apertures, called Venae vorticosae' by courtesy, into a 

 system of extra-ocular venous sinuses (Fig. 161, v, v) which fill the 

 orbital capsule and cushion the eyeball, much as does the orbital fat of 

 a higher vertebrate. These sinuses are present in brook lampreys also; 

 but here, the chorioid is usually no thicker than the pigment epithelium 

 of the retina, and indications of a subscleral sinus can be seen fairly 

 clearly only in such large species as Entosphenus lamottenii (= appendix). 



The iris has smooth inner and outer surfaces. The posterior layer of 

 its retinal portion contains pigment only in the parasitic species, and 

 then but little, mostly concentrated near the pupil. The anterior layer, 

 which in other vertebrates gives rise to the sphincter and dilatator pupil- 

 lae, is epithelial and heavily pigmented in all lampreys. This situation is 

 quite diagrammatically primitive, for the iridic continuations of the 

 retinal pigment epithelium and the sensory retina thus preserve their 

 respectively pigmented and unpigmented conditions in lampreys, instead 

 of exchanging them (contrast Fig. 7g, p. 15). The lamprey iris possesses 

 but little stroma, this in turn with little pigment or none. In brook forms, 

 there is just enough stromal tissue to hold together the thin layer of 

 blood vessels, which lies immediately against the retinal layers and forms 

 apparently the anteriormost tissue of the thin iris. Large lampreys how- 

 ever have a substantially thick argentea layer anterior to the blood-vessel 

 layer. It does not continue around the chorioid (c/. pp. 235-6). The 

 blood-vessel layer in all lampreys is much like a choriocapillaris; but it is 

 independently fed by three small, symmetrically-arranged arteries which 

 enter the eyeball anteriorly. 



