544 GASTROPOD/ [Opisthobmnchia. 



Rather large animals of flabby consistency, remarkable for the 

 4 large ear-like tentacles and high back, which have earned for them 

 the name of sea-hares. 



They are nearly world-wide in distribution in tropical and tem- 

 perate seas, and almost without exception inhabit shallow water. 

 Marine plants form their main food. Their colours 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 pig- 

 ment remains unchanged, so that markings of black or grey are per- 

 manent in specimens preserved in the ordinary manner. Being with- 

 out shelly armour, they are largely dependent upon imitative colour- 

 ing for protection ; this being supplemented by the ability to expel 

 a large amount of violet or purple fluid, darkening the water around 

 them, and also a milky fluid of nauseous odour ; this is secreted by 

 the " opaline gland " or " gland of Bohadsch." 



KEY TO GENERA. 



A. Anterior end of pleuropodial (dorsal) lobes well separated. 



Shell present, not calcareous . . . . . . . . TETHYS. 



B. Anterior end of pleuropodial lobes contiguous, the lobes not 



freely mobile. Shell a minute vestige or absent . . . . NOTARCHUS. 



Genus 1. TETHYS, Linne, 1758. 



Tethys, Linne, Syst. Nat., ed. x, 653. Type : T. leporina, L. Tethys, 

 Linne, 1758 ; not Linne, 1767, nor Tethys, Thetis, Thethys, &c., of sub- 

 sequent authors, to the present day. Aplysia, Linne, 1767, and of most 

 subsequent and all modern authors. Luplysia, Linne, 1767. Siphonolus, 

 A. Adams and Reeve, 1848. Syphonopyge, Keferstein, 1866. Neaplysia, 

 Cooper, 1863. Esmia, Leach, 1852. Lernea, Bohadsch, 1761 (not bi- 

 nomial). Dolabella, Risso, 1826, and in part of Lamarck, 1822. 



Animal swollen behind, narrower in front, with rather long neck 

 and head, bearing folded tentacles and slit rhinophores, as usual in 

 the family, the latter about midway between tentacles and dorsal 

 slit. Pleuropodia arising in front of the middle of animal's length, 

 ample, freely mobile, free throughout their length or united for a dis- 

 tance behind, functional as swimming-lobes ; anterior ends separated. 

 Mantle nearly covering the gill, having a median tube, foramen or 

 orifice communicating with shell-cavity, and produced behind in a 

 more or less developed lobe or lobes, folded to form an excurrent 

 siphon. Genital orifice under front edge of mantle, in front of gill ; 

 opaline gland present, a short distance behind genital opening. Foot 

 well developed. Radula with a large multicuspidate central tooth ; 

 lateral teeth with long cusps, serrate on both sides. 



Shell very thin, membranous, with a thin calcareous inner layer, 

 nearly as large as the mantle, concave, with pointed small apex, 

 bearing a recurved lamina, and having a concave posterior sinus. 



Distribution. All tropical and warm temperate seas. 



The animal not only crawls, but swims actively by means of the 

 " swimming-lobes." 



