166 COMPARATIVE ANATOMY. 



The sensory sacs of Ganoids, 1 which are confined to the head, and the 

 sensory tubes" of Elasmobranchs represent peculiar modifications of the 

 sensory organs in question. The former are sac-like invaginations of the 

 epidermis, while the latter have the form of delicate tubes, which give rise at 

 their base to one or more swellings or "ampullae." Both are lined by a 

 sensory epithelium of the same structure as that described on p. 164. 



As regards the function of these sensory organs, it can 

 only be affirmed that they must have to do with the percep- 

 tion of mechanical irritations from the surrounding water : 

 in what manner the impulses give rise to perception cannot be 

 definitely stated. In many cases they are thought to be con- 

 cerned with the perception of sound, and we shall see that this 

 is not improbable when we come to consider the auditory organ. 



The following is known with regard to the development of the lateral segmen- 

 tal organs. The dorsal roots of the cranial nerves ( F, F/7, F/Z7, IX, and X) 

 are during a certain embryonic period connected with the cells of the epiblast, 

 with which they become completely fused. Each of these masses then grows 

 and proliferates very rapidly, and the epiblastic thickenings thus resulting 

 represent the rudiments of the ganglia of the dorsal roots of the cranial nerves 

 as well as the first indications of the segmental sense-organs. Later, each 

 ganglion becomes separated from the skin, though it remains connected with 

 the corresponding sense-organ by means of a delicate nerve-fibre. A similar 

 mode of development also obtains in the segmental sense-organs of the trunk. 



It is a very interesting fact that in embryos of the sheep and cow, 8 to 

 10 mm. long, the ganglia of the facial, glossopharyngeal, and vagus are fused 

 with the epiblast, and thus indications of segmental sense-organs are still 

 present, though they disappear or become modified later. Beard has also 

 found rudiments of these organs in chicks of the third day. 2 



/>. End-Bulbs. 



In the organs described above (Fig. 132) great differences in 

 size and form between the central and peripheral cells may be 

 recognised : similar organs, however, exist near them in which both 

 kinds of cells are quite similar to one another in these respects. 

 These are the so-called end-bulbs. 



In all Fishes they are scattered irregularly over the whole body, 

 but especially over the head ; from the Amphibia onwards, they 

 are present in the mouth-cavity only, and are no longer seen 

 outside it. 3 In Amphibians they occur on the palate and on 

 the fungiform papillae of the tongue, and in Lizards and Blind- 

 worms they are also present on the inner sides of the upper and 

 lower jaws. In Mammals they are found on the soft palate, on the 

 walls of the pharynx, and on the epiglottis, and even extend into 

 the larynx ; but here also they are most constant and numerous on 

 the tongue, where they occur on the circumvallate and fungiform 

 papillae, as well as on the papilla foliata. 



1 Similar organs are present in Amiurus cattus. 



2 Beard proposes the name of branchial sense-organs for these structures, as 

 he considers them to be primitively the special sensory organs of the gill-clefts. 



3 They have also been found in the mouth and pharynx of Dipnoi. 



