[31] Langerhans _loc. cit._ describes the eye of the Ammocoetes in some respects very differently from the above. Very probably his description applies to an older Ammocoetes. 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. [FIG. 48. EYE OF AN AMMOCOETES LYING BENEATH THE SKIN. _ep._ epidermis; _d.c._ dermal connective tissue continuous with the subdermal connective 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 Descemet; _l._ lens; _v.h._ vitreous humour; _r._ retina; _rp._ retinal pigment.] 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 segmental tubes, which first appear in a larva of about 9 mm., becomes established behind the pronephros, and in an Ammocoetes 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 Planeri lives in the mud in streams. Without 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 August 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; and the dorsal fin becomes more prominent, and is divided into two parts. [FIG. 49. MOUTH OF PETROMYZON MARINUS WITH ITS HORNY TEETH. (From Gegenbaur; after Heckel and Kner.)] Besides these obvious external changes very great modifications are effected in almost all the organs, which may be very briefly enumerated. 1. Very profound changes take place in the skeleton. An elaborate system of cartilages is developed in connection with the mouth; the cranium itself undergoes important modifications; and neural arches become formed.