THE PINEAL EYES OF CYCLOSTOMES 189 



tissue which is especially well developed in Geotria, the New Zealand 

 lamprey (Fig. 134). 



This fills a gap in the cartilaginous roof of the skull. Above is a 

 layer of loose subcutaneous connective tissue and the epidermis. The 

 hypodermal cells of the latter are peculiar in containing no pigment 

 granules such as are present in this layer elsewhere, neither is there any 

 pigment in the subepidermal tissue. All the tissues between the parietal 

 organ and the superficial surface of the epidermis are translucent and 

 constitute the " parietal cornea " as defined by Studnicka. The pineal 

 organ can thus be seen through the cornea and appears as a central white 

 spot (Fig. 48, Chap. 3), as described on p. 71. 



Sections of the larger superficial vesicle (Fig. 45, Chap. 3) show that the 

 " eye " is of the ocellar or upright type. It consists of a superficial or 

 distal segment — the " lens " — termed by Studnicka the " pellucida " 

 and a deep or proximal segment, the retina. These are continuous with 

 each other at the circumference of the vesicle and they enclose between 

 them a central cavity filled with a loose syncytial tissue which in the living 

 animal is believed to enclose within its meshes a clear semi-fluid material. 

 The whole tissue with the viscous fluid contained in its spaces is termed the 

 vitreous. The cavity is in some specimens funnel-shaped (Fig. 45, Chap. 

 3, p. 69, and Fig. 14, Chap. 3, p. 19). The distal ends of the columnar 

 cells of the retina converge towards the lumen of the " groove " or " tube " 

 forming the narrow part or stalk of the funnel much in the same way as 

 they do in the cylindrical upright eyes of certain arthropods, e.g. Acilius 

 sulcatus, which in the large parietal eyes of young larvae, about 10-12 mm. 

 long show a central cleft in the retina ; this, as pointed out by Gaskell, 

 is strikingly similar to that in the parietal organ of Ammocoetes. The 

 cleft in the Acilius larva, moreover, is directly continuous with the 

 virtual cavity which occupies the central axis of the cylindrical " vitreous " 

 segment of the eye of Acilius sulcatus (Grenacher). It must be borne 

 in mind, however, that the bottom of the cleft in an Acilius or Dytiscus 

 larva (Fig. 38, Chap. 3, p. 53) corresponds to the bottom of the optic 

 pit, from which the eye is developed as a downgrowth of the surface 

 epithelium into the sub-epidermal tissue ; whereas in Ammocoetes the 

 blind end of the diverticulum from the roof of the interbrain is directed 

 distally away from the stalk, and the cleft or atrium is in the stalk near 

 the proximal end of the vesicle (Fig. 137, B). With reference to the 

 supposed origin of the funnel-shaped prolongation of the cavity of the 

 main vesicle into the distal end of the " optic stalk," it will be convenient 

 to allude here to the position and relations of the atrium, which is a 

 small accessory cavity in the wall of the main parietal organ. The 

 atrium varies considerably in size and shape in different specimens ; it 



