34 WORMS— MOLLUSCA. 



De Quatrefages, who compared them with the ciliated 

 wheels of Kotifers, and thought that they produced 

 currents in the water, thus urging microscopic algae, 

 infusoria, etc., to the mouth of the worm. Meyer, on 

 the contrary, with more probability, regards them 

 as olfactory organs. They are slight depressions 

 (Fig. 33) in the general surface, lined with peculiar 

 long cilice, supplied with a large nerve coming from 



'hp.dn: 



Fig. 33.— Section through the head segment of rolyophthalmus, x 300 (after Meyer). 

 Imd^ muscle; 6o, cup-shapfd organ; CM, cuticle; lip, hypoderm; Imd, longitu- 

 dinal dorsal muscle; n, peripheral nerve; C2. cnmmissure of brain; wif), mem- 

 branf ; ygn, pigment-cells ; h^dz, unicellular glands in the hypoderm ; gn, brain ; 

 k, nuclei in the braiu. 



the cerebral ganglion gn. Similar pits occur in many 

 other Annelida. They differ in number; Polyoph- 

 thalmus having only a pnir, the Capitellidae several. 



In the Mollusca, the hinder pair of tentacles have 

 been supposed by some to serve as olfactory organs. 

 In the cuttle-fish (Cephalopoda) there are certain pits, 

 at the base of which is a papilla, supplied with a nerve, 

 which is perhaps olfactory.* The true function of the 



* Leydig, " Histologie." 



