254 COMPARATIVE ANATOMY 



Pteraspidae : the so-called " spectacles " of Archegosaurus probably 

 belong to the same category. Their function is not thoroughly 

 understood, but there is no doubt that they are concerned with 

 the perception of mechanical stimuli from the surrounding water, 

 and are thus probably important in appreciating the direction of 

 these stimuli. As already mentioned (p. 242), they and the auditory 

 organ are genetically related to one another, the ear being merely 

 a specialised portion of the lateral line system. 



b. End-buds and gustatory organs. 



Although various intermediate forms between the nerve- 

 eminences and end-buds occur, it is an open question as to whether 

 there is any genetic connection between these two 'kinds of sense- 

 organs, and it is important to bear in mind that the nerve-supply 

 in the two cases is a very different one. 



In contrast to the nerve-eminences, which tend to sink below 

 the surface, the end-buds usually form a dome-like elevation pro- 

 jecting above the general level of the epiderm. A central sensory 

 epithelium, provided with sensory hairs, and peripheral supporting 

 cells can be recognised, but the former are as long as the latter. 

 These organs are supplied by the facial nerve. 



In Lampreys and most Elasmobranchs they remain at a primitive 

 stage of development, but become of great importance in Teleos- 

 tomes, in which they are scattered irregularly over the whole body 

 and are particularly numerous on the fins, lip-folds, barbules, and 

 mouth. 



From Dipnoans onwards they are limited to the oral, pharyngeal, 

 and nasal cavities, and are supplied by the IXth and Xth as well 

 as by the Vllth nerve. Thus in Amphibians they occur on the 

 papillae of the oral and pharyngeal mucous membrane, on the 

 margins of the jaws, and on the apices of the fungiform papillae of 

 the tongue, which may possibly have a gustatory function. 



In Reptiles the distribution of these organs is somewhat more 

 limited, and in Birds true taste-buds, though present in the mouth 

 and pharynx, are wanting on the horny tongue. The lingual 

 branch of the glossopharyngeal nerve is, however, strongly de- 

 veloped in many groups (e.g. Lamellirostres), and functionally 

 replaces the sensory branch of the trigeminal, which is wanting in 

 the avian tongue. 1 



1 In Lizards and Crocodiles typical gustatory organs are present, chiefly on 

 the soft, glandular mucous membrane of the pharynx : they are wanting on 

 the tongue and in the anterior part of the oral cavity. In Birds their dis- 

 tribution is apparently dependent on the size and form of the tongue : when the 

 latter is narrow, they are situated on the thin, glandular mucous membrane 

 of the lower beak ; and when it is broad, on that of the upper beak or of the 

 pharynx. Their arrangement is irregular, and their number varies greatly in 

 the different groups, being greatest in Parrots (300-400), in which their structure 

 resembles that of the gustatory organs of Mammals. 



