MOLLUl^CS. 



303 



About sixty species of Aphjsia are known from the whole world, though none are 

 found on our northern coasts. On the Portuguese shores thej' exist in large numbers, 



Fig. 345. — Ajjlt/sia depiUtns, sea-hare. 



and occasionally an easterly storm will throw them up in such quantities on the beach 

 as to cause an epidemic of sickness as well as to render the extraction of the purple 

 a matter of economic importance. 



The last family of the Opisthobranchs to lie mentioned is the Pleueobranchid.e, 

 represented on our coasts by the recently dis- _ ^ ..-j__ ^ ,_, 



covered lYOOnsia obesa. In all the inemliers - _ — "°°^ fca.jj 



of the family the u]iper jaw is wanting, the tjc; _- , -5;^,= ^ — ' - 



stomach very complicated, and divi<lcd into 

 several compartments. The shell, which is 

 usually present, is either borne on the back 

 like that of a limpet, or it is concealed as in 

 the typical genus Pleiirohraiiclms. These 

 forms, when creeping slowly through the 

 water, remind one of turtles, ami in some 

 the reseml)lance is strengthened by the dis- 

 tribution of color. In their living state most =|: j^^ 

 of the forms are very handsome. TJmhrelhi ^.^^. ^j,^ ^, 



is an aptly named genus, for the shell which 



covers tlie back bears no little resemblance to the familiar object bearing the same 

 name. 



Order II. — PULMONATA. 



The Pnlmonata or Pulmonifera is a group of terrestrial or fresh-water molluscs in 

 which respiration is effected by means of a lung or pulmonary sac, no gills being 

 developed. All the members are hermaphroditic, and an operculum is never formed. 

 Not all the land and fresh-water gasteropods are here included, for, as we shall see 

 further on, many families which have the same habits are entirely at variance with 

 the Pnlmonata in the essentials of structure — most prominent being, that, in the one, 

 the visceral nervous loop is straight; in the other, a twisted condition is found. 



