494 



MOLLUSCA. 



Class I. €I:PHAI^0I*0I>A. 



This tcrni is dorivctl t'loiii two (iroek words, h'plial<\ a head, and ^ww, a foot, and refers to the 

 fact th.ii thf aiiiiii.ils nl" liiis class ha\c the legs arranged around the head, and may therefore be 

 called hcad-foohd. These legs, it is true, are also arms, and being furnished with rows of suck- 

 ing-cups, enable these animals to maintain a iirni grasp of any object on which they lay hold. 

 They breathe by means u( branchiie connected with a funnel or siphon, through which the water 

 passes off. This arrangcuicnt answers a double purpose, for by expelling the water vigorously 

 through this siphon, ihey propel themselves rapidly along, this being their common mode of 

 swinnuiiig. They thus move backward, their eyes, however, being so situated as to command a 

 view of the objects in their course. Some species possess fins, and some use their arms to aid 

 them in swinuuing. In all the body i.s inclosed in a sac-like mantle, generally of thick, leathery 

 skin, covered, liowever, with a delicate cuticle, which in some species displays the most brilliant 

 colors. They arc all oviparous. This class is divided into two orders, the Dibranchiata and 

 Tetrabrauchiata. 



ORDER 1. DIBHANCHIATA. 



This term, derived from the Greek dis, two, and the Latin hranchice, giU^? signifies animals 

 having two gills. The order includes a great number of fossil as well as living species, dis- 

 tinguished for possessing two branchiae, and a strnctui'C of body which enables them not only to 

 swim rapidly through the water, but to creep with the head downward, upon the bottom of the 



^^-,^30^3^ sea. The shell is generally 



'i^'v^^^^-^^^r^^. internal, and often merely 



rudimentary ; if external 

 it is not chambered. The 

 arms arc eight or ten in 

 number, and furnished 

 \nX\\ sucking-discs, and 

 the body is often sup- 

 plied with a })air of fins. 

 They are divided into two 

 sections, the Octopoda axid. 

 Dccapoda. 



THE OCTOPODA. 



,,^f®« 



^'^^-. 



These: are furnished with 

 eight arms or legs ; two 

 of which, iu one genus, 

 enable the animals to form 

 shells, to which, however, 

 the body i.^, not attach- 

 ed; others are naked, 

 having a sliglit trace of an 

 internal slu'll. They arc 

 \ for the most part exceed- 

 ingly active and voraci- 

 \ ous, and prey with avidity 

 upon Crustacea and fishes. 

 They arc provided with 

 a sac containing a black 



and acrid fluid, which they eject into the water so as to obscure it and render it offensive, by 



means of which they escape from their enemies. 



AHGOXAUT WITn THE PHELL. 



