CYGLOSTOMATA. 



81 



constitute the commencement 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 (I) has the normal structure of the embryonic 

 lens of Yertebrata. 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 

 ( i'/i). 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 (1) the epidermis (ep); (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 repre- 

 sents Descemet's membrane. The sub-dermal connective tissue is continued 

 as an investment round the whole eye. There is no specially differentiated 

 sclerotic, and a choroid is only imperfectly indicated 1 . The peculiar 

 features of the eye of the young larva of the Ammoccetes are probably due 

 to degeneration. 



In the brain the two cerebral hemispheres lie one on each side of the 

 anterior end of the thalamencephalon. There are well-defined olfactory 

 lobes, and two distinct olfactory nerves are present. 



The excretory system has undergone great changes. A series of seg- 

 mental tubes, which first appear in a larva of about 9 mm., becomes 

 established behind the proiiephros, and in an Ammocietes of 65 mm. the 

 pronephros has begun to atrophy. The generative organs are formed in a 

 larva of about 35 mm. Shortly before the metamorphosis the portion of 

 the cloaca into which the segmental tubes open becomes separated off as a 

 distinct urinogenital sinus, the walls of which become perforated by the 

 two abdominal pores. 



The Ammocoetes of Petromyzon Pla- 

 neri lives iu the mud in streams. With- 

 out undergoing any marked changes in 

 structure it gradually grows larger, and 

 after three or four years undergoes a 

 metamorphosis. The full-grown larva may 

 be as large or even larger than the adult. 

 The metamorphosis takes place from Au- 

 gust till January. The breeding season 

 sets in during the second half of April; 

 and shortly after depositing its generative 

 products the Lamprey dies. The changes 

 which take place in the metamorphosis 

 are of a most striking kind. 



The dome-shaped mouth of the larva 

 is replaced (fig. 47) by a more definitely 

 suctorial mouth with horny cuticular teeth 

 (fig. 49). The eyes appear on the surface ; 



FIG. 49. MOUTH OF l'i;- 



TKOMYZON MABINUS WITH ITS 



HOKNY TEETH. (From Gegeii- 

 banr; after Heckel and Kner.) 



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 (1) that the 

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



15. E. II. 



6 



