60 APLYSIID^E. 



them the name of Sea Hares. They are nearly world-wide in dis- 

 tribution in tropical and temperate seas, and almost without excep- 

 tion inhabit shallow water. Marine plants form their main food. 

 Their colors in life are often bright and variegated, but in alcohol 

 the green and violet tints are evanescent, fading usually to a dirty 

 light olive ; but the black pigment remains unchanged, so that 

 markings of black or gray are premanent in specimens preserved in 

 the ordinary manner. 



Being without shelly armor, AplysiidcB are largely dependent 

 upon imitative coloring for protection ; this being supplemented by 

 the ability to expel a large amount of violet or purple fluid darken- 

 ing the water around them, and also a milky fluid of nauseous odor. 



Only one species has been known to be utilized by man : 

 Dolabella teremidi being esteemed and largely used for food by the 

 natives of Borabora. 



The means of locomotion are varied : Tethys not only crawls, but 

 swims actively by means of the muscular, wing-like pleuropodia, 

 or " swimming lobes ; " Dolabella, Petalifera, Phyllaplysia and other 

 genera with largely united pleuropodia, are restricted to creeping 

 like ordinary snails; Notarchus, which lives exclusively upon float- 

 ing sea-weeds, has a narrow foot adapted to travelling along their 

 slender stems, but has also been observed to dart rapidly by a forci- 

 ble expulsion of water from the large gill cavity by contraction of 

 its enclosing walls. This method, very exceptional in a gastropod, 

 is quite analogous to that of the squids. 



Notes on external and internal anatomy. 



The notes following are mainly restricted to features useful for 

 purposes of classification, the limits of this work denying space for 

 any thorough discussion and illustration of the anatomy and histol- 

 ogy of the group. 



The main external features of Aplysiidce are shown by the figures 

 and diagrams on plates 35 and 66. The head and tentacles are 

 sufficiently shown in pi. 35, fig. 31 ; to the right of the right rhino- 

 phore or posterior tentacle is seen an eye, and the genital groove ; be- 

 hind are seen the two pleuropodia or swimming lobes, one folded 

 over the back, the other spread ; within these is the oval mantle, 

 the smooth inner portion of which encloses the shell, to which a 

 median pore, the mantle foramen, opens ; on the right is seen the 

 free margin of mantle, which is fleshier, and contains along its inner 



