TECTIBRANCHIATA ; APLYSIAD.E ; BU 



387 



These animals feed on sea-weed. They are very sluggish in 

 their movements, but have a peculiar means of defence, con- 

 sisting of a deep purple liquid (said by some to have acrid pro- 

 perties), which they can discharge from the edge of the mantle 

 when alarmed, and by which the surrounding water is dis- 

 coloured, so that the animal cannot be discerned. Amongst the 

 singular animals referred to this family, one of the most remark- 

 able is the Bursatella (Fig. 677), an inhabitant of the Indian Seas. 

 Its gills project far beyond the opening of the mantle. Nearly 

 allied to the Aplysia are the Bulla and Bullcea, which belong 

 to the family BULLID^E. These have a small calcareous shell, in 



which the spiral form be- 

 gins to manifest itself. 

 The Bullcea apcrta (Fig. 

 675) is found in almost 

 every sea, living on oozy 

 bottoms. The Bulla lig~ 

 naria (Fig. 676, a) is re- 

 markable for the density 

 of the walls of the sto- 

 mach ; amidst the tendin- 

 ous fibres of which, a large 

 quantity of calcareous 

 matter is deposited, form- 

 ing plates of bony firm- 

 ness ; these are moved 

 against each other by 

 powerful muscles, so as to 

 rub down almost any sub- 

 stance that is placed between them. The Author has more than 

 once found a small bivalve shell in this situation. The shell of 

 the Bulla ampulla (Fig. 676, b) is interesting, as exhibiting, in 

 its flat open form, a transition towards that of the Aplysia. In 

 juxtaposition with the Bullidaa, some writers place the TORNA- 

 TELLID^E, which are distinguished from all the other species 

 of the Order by their regularly spiral external shell. 



1004. The PLEUROBRANCHID.E (Fig. 653) are also gener- 



FIG. 677. BUR.SATKI.I.A LfUCHIl. 



