184 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



INTRODUCTION. 



The Mollusc of which the present Memoir is the subject 

 belongs to the Euthyneura, a sub-class of the Gastropoda. 

 The Euthyneura are hermaphrodite, and usually, except in 

 primitive forms, have undergone detorsion so that the visceral 

 nerve cords are uncrossed. In many cases a secondary external 

 symmetry has been acquired. The Euthyneura are divided 

 into two orders, Opisthobranchiata, including Aplysia, and 

 Pulmonata, including land forms such as the Snail. In Opis- 

 thobranchiata the ctenidium is situated posterior to the heart, 

 and there is usually a widely-open pallial cavity. The cteni- 

 dium is covered by the overhanging mantle in the sub- order 

 Tectibranchiata, containing Aplysia, but this mantle skirt is 

 absent in the Nudibranchiata. 



The family Aplysiidae has six genera, viz. : — Aplysia, 

 Dolabella, Dolabrifer, Aplysiella, Phyllaplysia, and Notarchus. 

 Of these only Aplysia is British, and of the three best known 

 European species, Aplysia limacina, A. depilans, and A. 

 punctata, only the last named inhabits the waters round our 

 shores.* 



The popular name for Aplysia is the Sea-hare ; it has 

 been called this since the days of Pliny owing to its resemblance 

 to a sitting hare when contracted, and to the similarity between 

 the auriculate tentacles and the ears of the hare. The name 

 Aplysiaf was first given to the Sea-hare in the 13th edition of 

 the " Systema Naturae " of Linnaeus (1791), though the 12th 



* It was formerly supposed that the large Aplysiae, taken in Torbay 

 by Major Hunt in 1875 (" Trans. Devonsh. Nat. Assoc.," 1877). were 

 specimens of A. depilans, but the characters of the species were not then 

 sufficiently well denned to determine this with certainty. Major Hunt 

 later (1904) considered them unusually large specimens of A. punctata, and 

 for many years only this species has been known on British coasts. Similarly 

 the very small rose red species of Thompson has been shown to be the 

 young stage of A. punctata. 



f The Aplysia mentioned by Aristotle is not a Mollusc, but a Sponge. 

 The word means " that which cannot be washed " or possibly " washed 

 with." 



