9 8 



METAMORPHOSIS. 



The olfactory sack is nearly divided into two by a ventral septum. The 

 eye (fig. 48) is much more fully developed, but lies a long way below the 

 surface. The optic cup forms a deep pit, in the mouth of which is placed 

 the lens. The retinal layers are well developed (cf. Langerhans), and the 

 outer layer of the optic cup or layer of retinal pigment (rp} contains 

 numerous pigment granules, especially on its dorsal side. At the edge of 

 the optic cup the two layers fall into each other. They constitute the com- 

 mencement of the pigment layer of the iris ; but at this stage they are not 

 pigmented. The mesoblast of the iris is hardly differentiated. The lens (/) 

 has the normal structure of the embryonic lens of Vertebrata. The inner 

 wall is thick and doubly convex, while the outer wall, which will form the 

 anterior epithelium, is very thin. There is a large space between the lens 

 and the retina containing the vitreous humour (v.h\ There is no aqueous 

 humour, and the tissues in front of the lens bear but little resemblance to 

 those in higher Vertebrata. The cornea is represented by (i) the epidermis 

 (<?/) ; (2) the dermis (d.c) ; (3) the sub-dermal connective tissue (s.d.c) which 

 passes without any sharp line of 

 demarcation into the dermis ; (4) 

 a thick membrane continuous with 

 the choroid which represents Des- 

 cemet's membrane. The sub-der- 

 mal connective tissue is continued 

 as an investment round the whole 

 eye. There is no specially differ- 



s.d.c 



cfm 



entiated sclerotic, and a choroid 

 is only imperfectly indicated 1 . 

 The peculiar features of the eye 

 of the young larva of the Ammo- 

 ccetes are probably due to degen- 

 eration. 



In the brain the two cerebral 

 hemispheres lie one on each side 

 of the anterior end of the thala- 

 mencephalon. There are well- 

 defined olfactory lobes, and two 

 distinct olfactory nerves are pre- 

 sent. 



The excretory system has 

 undergone great changes. A series 

 of segmental tubes, which first 

 appear in a larva of about 9 mm., 



FIG. 48. EYE OF AN AMMOCCETES LYING 

 BENEATH THE SKIN. 



ep. epidermis ; d.c. dermal connective 

 tissue continuous with the sub-dermal con- 

 nective tissue (s.d.c), which is also shaded. 

 There is no definite boundary to this tissue 

 where it surrounds the eye. 



m. muscles ; dm. membrane of Desce- 

 met; /. lens; v.h. vitreous humour ; r. retina; 

 rp. retinal pigment. 



1 Langerhans loc. cit. describes the eye of the Ammoccetes in some respects very 

 differently from the above. Very probably his description applies to an older 

 Ammoccetes. The most important points of difference appear to be (i) that the 

 vitreous humour is all but obliterated ; (2) that the iris is much better developed. 



