436 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



way of the structure of the spines of Echinids. Their 

 small size and protected position, under large spines 

 or in special cavities of the test, prevent us from 

 regarding them as tactile organs, while their constant 

 approximation to the entrance into the digestive 

 tract justifies us, at present, in ascribing to them the 

 function of testing the food which is found in the 

 water in which their possessor lives. 



Very little is definitely known as to the organs of 

 taste in other Invertebrata, although, of course, most 

 do, on observation, exhibit some kind of preference 

 for certain foods ; this was seen by Mr. Darwin even 

 in the omnivorous earthworm. In Insects the 

 maxillary palpi are probably the seat of the organ, 

 and Lowne has described those of the blowfly as 

 having their cavity filled with cells, which are supplied 

 by a branch from the great nerve trunk of the 

 proboscis. Freely projecting epithelial papillae, not 

 unlike the gustatory organs of tadpoles, have been 

 observed on the tentacles of various Molluscs ; the 

 cells of which these papillae are composed are ciliated, 

 and appear to be well supplied with nerves ; their gus- 

 tatory function seems to have been demonstrated. 



Nothing is certainly known as to gustatory organs 

 in the Uroehordata or Cephalochordata. In 

 Fishes, the organs of this sense are only feebly 

 developed, and, as often happens with organs in a 

 generalised condition, they are not so definitely localised 

 as in the higher forms. The cup-shaped organs have 

 at their edge long cylindrical cells, with more delicate 

 cells in the central portion ; they are not confined to 

 the cavity of the mouth, but are found also on the 

 skin (compare the account of the teeth of Elasmo- 

 branchs, page 141); those that are placed on the 

 mucous membrane of the palate are supplied with 

 branches from the glosso-pharyngeal nerve. In the 

 carps they are described as being most largely developed 



