TECTIBRANCHIATA. BULL^IA ; BURSATELLA. 



377 



form begins to manifest itself. The Bullcea aperta (Fig. 573) 

 is found in almost every sea, living on oozy bottoms. The 

 Bulla lignaria (Fig. 574, a) is remarkable for the density of the 

 walls of the stomach ; amidst the tendinous fibres of which, a 

 large quantity of calcareous matter is deposited, forming plates 



of bony firmness; these 

 are moved against each 

 other by powerful mus- 

 cles, so as to rub down 

 almost anysubstance 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. 

 574, b) is interesting as 

 exhibiting, in its flat open 

 form, a transition to- 

 wards that of the Aply- 

 sia. To this group is 

 FIG. 575. BURSATELLA LEACHH. also referred the curious 



Bursatella (Fig. 575), 



which is an inhabitant of the Indian seas. Its gills project far 

 beyond the opening of the mantle. 



ORDER V. PECTINIBRANCHIATA. 



918. This Order is not only by far the most numerous in 

 the whole class, but contains the animals which may be regarded 

 as its most characteristic examples. They have all two tenta- 

 cula, and two eyes ; the latter being sometimes mounted on a 

 footstalk, as in the Snail. The mouth is prolonged into a sort of 

 proboscis; and the tongue is furnished with little hooks, or 

 recurved spines, which enable it to wear down the hardest bodies 

 by slow and oft-repeated action. The cavity, in which the gills 



