MOLLUSCA. 225 



reversed shells are spoken of as sinistral (left- 

 handed), because the spiral, when traced up towards 

 its commencement, turns from the right towards the 

 left. An instance is afforded by the Spindle-shell 

 {Fusus antiquus), the reversed variety of which has 

 received the name of Fasus contrarius. Sinistral 

 shells occur also among the Cephalopoda (see fig. 

 78, p. 227). 



The eggs of the univalve molluscs are often covered 

 with a leathery egg-shell. The eggs of the large 

 Whelk {Buccinum) which form large cellular masses, 

 and those of the small Whelk {Purpura), which are 

 egg-shaped and attached by the thin end to stones, 

 etc., are both common objects of the shore. 



The Gastekopoda include the ProsobrancJdata, with 

 gills in front of the heart, an order which contains the 

 majority of marine univalves, and includes all those 

 which have been already mentioned ; the Heteropoda, 

 deep-sea forms with a small rudimentary shell; the 

 Pulmonata, or Snails; and the Opisthohrancldata, or 

 Sea-snails. The latter include a number of shell-less 

 marine forms, and also the " bubble-shells," of which 

 several are found on our shores. The Placophora, 

 sometimes classed with the Prosobranchiata, are a 

 primitive form belonging to the Gasteropod: one 

 representative, Chiton, may be found adhering to 

 rocks. It has a series of eight calcareous plates 

 situated down the back. It possesses a well-de- 

 veloped odontophore. 



