200 



SYSTEMATIC ZOOLOGY. 



ORDER II. MONOTOCARDIA. 



Here belong the great majority of marine snails, all of 

 which agree in having but a single gill and a single auricle 



to the heart. Few of them have any 

 economic interest aside from those which 

 feed upon oysters and other valuable 

 shell-fish. These injurious forms com- 

 monly known as ' drills'- -are able to 

 bore holes through the shells of oysters, 

 etc., by means of their lingual ribbons. 

 Many, however, are great favorites with 

 collectors, among them the strombs 

 (fig. 38), cones, cowries, and olives. 

 Some of the cones are noticeable from 

 the fact that they have a poison-gland 

 connected with the lingual ribbon. Some species, for- 

 merly grouped as a distinct order of Heteropoda, are espe- 

 cially modified for a life on the high seas. 



SUBCLASS II. EUTHYNEURA. 



In the Euthyneura the nervous system is without a 

 twist, and the head almost always bears two pairs of 

 tentacles. 



ORDER I. OPISTHOBRANCHIA. 



These forms are all marine, and have but two divisions 

 to the heart an auricle and a ventricle, the latter being in 



FIG. 38. S t r o m b 

 (Strombus pugilis}. 

 After Woodward. 



FIG. 39. Naked mollusc (Doris}, showing the gills, above to the right. 



front of the former. Some are provided with a spiral 

 shell, while others called Nudibranchs or naked mol- 



