ACT.EONIDjE. '6 



may be seen, its back splendid with varied eolours, or the 

 Operculatunij more sedentary in its habits, fastened, by its 

 thick, deep, orange foot studded with pearl-like tubercles, 

 to the bottom of the shallow pools ; while gliding briskly 

 along the branches of the corals the Aplys'uc or " Sea 

 Hares " may be noticed, extending their necks, and busily 

 exploring everything within their reach. The gills in 

 this Order are un symmetrical, more or less covered by 

 the mantle and protected by a shell, in some families 

 being situated on the hinder part of the back, and in 

 others at the side of the body under the margin of 

 the mantle. The sexes are united, the male and female 

 organs existing in the same individual, so that reciprocal 

 impregnation is necessary to reproduction. The shell is 

 either external and spiral, and provided with an opercu- 

 lum, as in Act (eon ; conical and external, as in Tylodina 

 and Operculatum; rudimentary, and contained in a fold 

 of the mantle, as in Philine and Aplysia; or it is altoge- 

 ther wanting, as in Bursatella and Runciria. The eyes 

 are sessile on the head, and the tentacles are either auri- 

 form and folded on themselves, as in the Aplysiida, or are 

 united to form a broad cephalic disk, as in the Bull idee. 



Fam. ACT^EONIDiE. 



Teeth, central none, lateral numerous, uncinated, in a 

 diverging cross series. Head depressed, forming a 

 quadrate disk, bi-lobed in front, with broad, posterior, 

 tentacular lobes ; eyes sessile on the middle of the head. 

 Mantle included within the shell ; branchial plume single. 

 Foot oblong, truncate in front, obtuse behind. 



Operculum horny, linear, transverse. 



