MOLLUSKS. 155 



sands of sea-shores or the muddy bottom. The Arenicola family includes species 

 that burrow in the sands of sea-shores. Fig. 170 represents the A. piscatorum, or 

 Lob-worm, which is common on European shores, and grows to the size of the 

 finger. One species of Eunice has a length of four feet. 



(2.) The Tubicola, or Serpula tribe, which live in a calcareous or membranous 

 tube and have a delicate branchial flower, often of great beauty, near the head. 

 They are confined to salt water. The tubes often penetrate corals, and the 

 branchial flower comes out as a rival of the coral polyps around it. 



(3.) The Terricola (Oligochseta), or Earth-worm tribe, destitute of branchial 

 appendages ; as the common Earth-worm. 



(4.) The Suctoria, or Leech tribe; sucking-worms; as the Leech. 



Besides these there are the Helminths, including most Intestinal worms, and 

 the Turbellaria. 



III. MOLLUSKS. 



The Ordinary Mollusks are usually divided into — 



(1.) The Acephals, or headless Mollusks ; as the Oyster and Clam; 



(2.) The Cephalates, having a head ; as the Snail; and, 



(3.) The Cephalopods, having the head furnished with feet; as the 

 Cuttle-fish, 



The headless species have a mouth, but no perfect organs of 

 sight ; the Cephalates have distinct eyes and a distinct head (fig. 

 166) ; the Cephalopods have the eyes large, and can grasp with 

 great power by means of their arms (fig. 169). These arms corre- 

 spond to mouth-appendages or palpi (feelers) in other Mollusks. 



The fleshy body of Mollusks has on either side a loose skin or 

 fleshy leaflet starting from the back, which covers the sides of the 

 body like a cloak, and is either open or closed along the venter : it 

 is called the -mantle or pallium (cloak). This mantle lies against the 

 shell in the oyster, clam, and allied species, and secretes it ; and in 

 the univalves it is reflexed over more or less of the exterior of 

 the shell, and performs the same function. It enables the animal 

 to give the ornament in color and form which is found over the 

 exterior of many univalves. 



1. Cephalopods, or Cuttle-fish tribe. — The shells of this tribe 

 are distinguished almost invariably by having transverse partitions, 

 — whence they are called chambered shells (fig. 168). They may be 

 either straight or coiled ; but with few exceptions they are coiled 

 in a plane, instead of being spiral. A tube, called a siphuncle, 

 passes through the partitions ; and this siphuncle may either 

 be central or nearly so, as in the genus Nautilus (fig. 168), or lie 

 along the inner or ventral side of the cavity, or the outer or dorsal 

 side, as in Ammonites. The animal occupies the outer chamber,, 

 as in fig. 168. 



