MOLLUSCS. 



303 



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

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



Fig. 345. — Aplysia depitans, 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 be mentioned is the Plbtjeobeanchidje, 



represented on our coasts by the recently dis- _ _^^ _ ^^ 



covered JToonsia obesa. In all the members - ^==^ "^ ^ ^ ^^ 



of the family the upper jaw is wanting, the 

 stomach very complicated, and divided 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 Pleurohranchus. These 

 formsj when creeping slowly through the 

 water, remind one of turtles, and in some 

 the resemblance is strengthened by the dis- 

 tribution of color. In their living state most 

 of the forms are very handsome. Umbrella 

 is an aptly named genus, for the shell which 

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

 name. 



Order II. — PULMONATA. 



The Pulmonata 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 Pulmonata 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. 



Fig. 346. — Pleurohi-anchus peronii. 



