PHYSIOLOGY OF THE INVERTEBRATA. 353 



the form of papillae or prorainences covered with hairs, or of 

 tubular prolongations of the skin. In some of these animals 

 there is a sac filled with calcareous granules attached to the 

 ganglion. This sac is most likely an auditory organ. 



One or more ocular spots are sometimes situated on the 

 ganglion in the Botifera. 



In some of the Trematoda, ocular spots have been observed, 

 but no other sense-organs. 



The Annelida. 



In these animals the sense-organs are variously distributed. 

 The organs of touch are cutaneous, and they have the form of 

 bristles (setae), &c., in connection with sensitive fibres. 

 According to Leydig, these tactile organs are sometimes, in 

 the Hirudinea, grouped in large numbers at the bottom of 

 cup-shaped depressions. In Sirvdo, there are about sixty of 

 the cup-shaped depressions in the head, and others in the 

 posterior part of the body. They are in connection with the 

 terminations of nerves given o£E to those in the head from the 

 supra-cesophageal ganglia, and to those posteriorly situated, 

 from the caudal ganglion. These organs are also of an 

 olfactory function (Leydig). 



In the Gephyrea rudimentary or incipient eyes are some- 

 times connected with the cerebral ganglion. Simple eyes 

 are usually present on the anterior segment in the Hirudinea. 

 These are supplied by nerves from the supra-oesophageal 

 ganglia. 



In Hirudo the eyes are situated on the dorsal surface of 

 the first three segments. 



In Lumbricus (one of the Oligochceta) no eyes or other 

 special sense-organs are present. Although devoid of sense- 

 organs, Zumbricus " possesses a generalised sensitiveness, due 

 to the plentiful distribution of nerve-fibres through the body, 

 and which, in many respects, takes the place of a series of 

 specialised organs, corresponding to the senses of touch, taste, 



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