112 MANUAL OF NATURAL HISTORY. 



tunic or mantle, which often secretes a shelly coat 

 to guard the more delicate organs from external 

 violence. These shelly envelopes are the prized, 

 and much sought for, objects of the Conchologist, 

 and are alike remarkable for their imperishable 

 nature, the extreme beauty and elegance of their 

 forms, the richness of their colouring, and the deli- 

 cacy and endless diversity of their sculpture and 

 markings. The molluscous architects of these cal- 

 careous homes are generally apathetic beings, dis- 

 playing but a small amount of energy when con- 

 trasted with the restless, active, air-breathing legions 

 of the articulated classes. 



They comprise the Cephalopoda, the highest of 

 their class, with prehensible arms disposed around 

 their mouth, large and perfect eyes, and with the 

 sexes separate ; the Pteropods, with their bodies 

 often enclosed in fragile shells of glassy transpa- 

 rency, and with fin-like expansions on the sides of 

 their heads to enable them to navigate the high 

 seas ; the Gasteropoda, furnished with a muscular 

 disk on the lower portion of their bodies adapting 

 them for crawling on the ground, and with distinct 

 head, eyes, and tentacles ; the Bivalves, protected 

 by solid shelly plates, headless, and breathing by 

 means of plait-like gills ; the soft, aquatic, headless 

 Tunicaries, either free and isolated, or united or- 

 ganically in a common brotherhood ; and the Brach- 

 iopods, enclosed in bivalve shells, breathing by the 

 mantle, and with long spiral or twisted arms on the 

 sides of the mouth, by the ciliary movements of 



