Invertebrate Gallery of the Indian Museum. 127 



in a modified form, and is sometimes altogether absent : 

 when present it arises behind instead of beside the heart. 

 The Opisthohranchia are all marine : the majority are 

 found either in the tidal pools of reefs, or not far below 

 low-water mark ; a few are pelagic, inhabiting floating 

 sea-weed ; and a very few have been dredged from a con- 

 siderable depth in the ocean. The reef forms, especially 

 those of the tropical seas, are often of remarkably fantas- 

 tic beauty of form and splendour of coloration. 



In habit the Opisthobranchia vary, many being ex- 

 tremely rapacious, while others feed entirely on sea-weed. 

 Several of the tropical forms have the power of emit- 

 ting — probably for protection against enemies — acrid and 

 offensive odours ; and the Sea-hare when molested can 

 secrete an abundant cloud of purple fluid for purposes 

 of concealment. All these secretions probably come from 

 glands in the skin. All the Opisthobranchs are herma- 

 phrodite. 



The Opisthobranchs are divided into two sections, 

 namely, (i) the Palliata^ or Tectibranchiata, in which 

 the mantle and generally a thin shell are present cover- 

 ing the breathing-organs ; and (2) the Slug-like Non- 

 palliata, or Nudibranchiata, in which, in the adult, the 

 mantle and shell are wanting, leaving the gill-plume, when 

 it exists, exposed. 



/3. OPISTHOBRANCHIA PALLIATA. 



[OtaseB 167JI-C!!]. 

 There are two sub-Orders of mantle-bearing Opis- 

 thobranchs, namely, (i) the Ctenidiobranchia, which have 

 a functional gill-plume and usually a thin shell ; and (2) 

 the shell-less Phyllidiobranchia, in which the place of 

 the gill-plume is taken by a pair of fringes on the inside 

 of the mantle. The Phyllidiobranchia, which include but 

 one small family, are represented in the Case by drawings. 

 The Ctenidiobranchia, which comprehend (i) the Torna- 

 ellida.', (2) the large family of the Bullida' or " Bubble 



