VERTEBRATES. 



35 



surrounding nervous matter corresponds to the suprtt- and infra-CESophageal ganglia, 

 with their connecting commissures in the arthropods and annelids. There is little 

 evidence to support this theory ; it is far more probable that both epiphysis and hypo- 

 physis are rudiments of sense-organs which have become lost in the higher forms. It 

 has been suggested that the epiphysis is a rudiment of an unpaired eye, projecting 

 upwards from the thalamoccele, as do the optic vesicles laterally, and it is probable that 

 the hypophysis is comparable to certain sense organs possessed by Amphioxus and 

 the tunicates. The connection of the hypophysis with the mouth is very early lost, 

 the duct disappearing entirely except in the Myzonts, where, in the most remarkable 

 way, the ventral end of the duct shifts its position so as to open on the upper surface 

 of the head instead of in the mouth. A further reference to this condition will be 

 found in the description of the olfactory organs of these forms. 



THE HIGHER SENSE-ORGANS. 



The Auditoet Orgais-. 



All the higher sense-organs may be regarded as differentiated parts of the skin, 

 the supi:)lying nerves of which have become greatly specialized, and have thus acquired 

 a more marked individuality than the other sensory nerves. The ends of the nerves 

 are to be sought for in modified tracts of the epiblast, known as neuro-epithelial tracts, 

 where they terminate in sensory cells, generally isolated from 

 each other by indifferent cells. In the case of the olfactory 

 and auditory organs these neuro-epithelial tracts are developed 

 from hollow ingrowths of the ordinary epiblast of the head, but 

 in the case of the eye the neuro-epithelium (retina) is formed 

 as an outgrowth of the first cerebral vesicle, which, however, 

 it must be borne in mind, is epiblastic in origin. 



Surrounding the epiblast destined to form the neuro-epi- 

 thelium there is always some mesoblastic tissue which carries 

 the necessary vessels for the nourishment of the neuro-epithe- 

 lium, and which likewise forms a more or less comj)lete cap- 

 sule for the organ. We have already seen what an important 

 jiart these sense-capsules play in modifying the skull. 



A fish forms a convenient starting-point for the study of 

 the ear, because only the internal ear or auditory organ proper 

 is present, without the additional drum cavity and external 

 ear which we find in higher forms. The epiblastic ingrowth is at first sac-like 

 (Fig. 40), but soon gives out processes which result in the highly complicated 

 structure known as the auditory labyrinth. In sharks the auditory labyrinth is closely 

 surrounded by the cartilage of the skull on every side, but in teleosts it is to a great 

 extent free within the cranial cavity, owing to deficiencies in the inner wall of the cap- 

 sule. It is consequently very easy to dissect out the labyrinth of an ordinary bony 

 fish. The cat-fish will serve as an accessible type (Fig. 40). Here we recognize two 

 parts, an upper and a lower ; fhe upper composed of a vertical tube, the ' utriculus,' 

 with a large anterior i-ecess, with which are connected three semicircular canals, the 

 lower of a less capacious ' sacculus,' also provided with a recess, — • the ' cochlea.' Con- 

 necting the utriculus and sacculus is a narrow tube which in many cases is completely 



Fig. 39. — Head o£ embryo 

 fish {^Ctenolabrus); 6, brain ; 

 e, invagination for ear ; I, 

 Uiickening of epiblast for 

 the lens of the eye ; «, nasal 

 pit ; r, ?-', outer and inner 

 layers of optic vesicle 

 which later form the retina. 



