294 SCOTT. [Vol. I. 



separates it from the epiblast, and its position becomes gradually 

 deeper as the growth of the optic nerve and vesicle does not 

 keep pace with the increasing thickness of the walls of the 

 head. In Ammocoetes of 25-30 mm. in length a thick mass of 

 loose connective tissue occurs between the dermis and the eye. 



I have found none of the eye-muscles in the larvae, though 

 in young larvae the first head-cavity (Fig. 39, PI. XI) is seen 

 lying in close contact with the optic vesicle, and probably gives 

 rise to at least some of the muscles of the eye. 



At the time of metamorphosis the eye increases enormously 

 in size and approaches the surface ; the retinal elements rapidly 

 make their appearance, and the iris and cornea are formed. 

 Even the adult eyes shows some peculiarities, which Langerhans 

 has described, and into which it is needless to enter here. 



4. TJie Auditory Organ. — The structure of -this organ in 

 the adult has been described by J. Miiller, and latterly with 

 great minuteness by Ketel (21) and Retzius (29). It is in many 

 respects widely divergent from the plan exhibited in the higher 

 vertebrates, and the homologies of its various parts are matters 

 of great obscurity. The membranous labyrinth consists of a 

 vestibule, two semicircular canals, and a sac-like appendix of 

 the vestibule. The vestibule is of an irregular oval shape, with 

 the long axis placed antero-posteriorly ; it is divided by a rather 

 deep median fold into an anterior and posterior portion, which 

 are of similar size and appearance. This fold is seen in trans- 

 verse section to form a prominence in the interior of the laby- 

 rinth, and is crossed by a similar though smaller process, which 

 divides the vestibule into upper and lower "portions. (In P. 

 Planeri I have observed only doubtful indications of this sec- 

 ond fold.) These prominences are somewhat stiffer than the 

 other parts of the vestibule, and are covered with a thickened 

 and sometimes many-layered epithelium, some of the cells of 

 which are provided with stiff auditory hairs. Above each of 

 the divisions of the vestibule lies a semicircular canal, which is 

 but incompletely separated from the vestibule. The anterior 

 canal arises from the lower antero-external corner of the vesti- 

 bule, the posterior from the postero-external corner ; the two 

 canals pass upwards and inwards, and unite in the median dor- 

 sal line of the inner edge of the vestibule, and open into the 

 latter by means of a common passage. The external origin of 



