CHAPTER XL 



Mollusca(" Shellfish"). 



MOLLUSCA (Latin mollis, " soft ") means " soft-bodied," and the name is 

 applied to a group of backboneless animals, numbering at least 50,000 species, 

 the vast majority of which have no internal skeleton whatever. Instead, the 

 body is enclosed in a hard shell, which serves the purpose of an external skeleton, 

 and acts principally as a protection for the body. 



Most of the molluscs belong to the sea, but a considerable number inhabit 

 fresh water, and some, like the snail and slug, live upon the land. 



They possess no limbs in the ordinary sense of the word, but their soft 

 bodies are covered with a loose, thick skin, overlapping the sides and usually 

 very slimy, called the mantle, and by its means the animal secretes the lime 

 from which the shell is made. Another characteristic structure is the " foot," 

 a muscular protrusion of the surface of the belly, by which the creature usually 

 moves. 



There are also remarkable sense-organs tentacles like those of the octopus 

 and cuttle-fish and the horns of the snail; eyes, such as the well-developed 

 eyes of the cuttle-fish ; a pair of hearing organs, called ear-sacs, situated in front 

 of the foot ; and also a smelling organ, or scent-patch, at the base of the gills. 



Many of the molluscs have in the mouth region a toothed ribbon, tongue, 

 or palate, like a file, by means of which they rasp the plants, etc., on which they 

 feed. 



Their blood is colourless, or with a light blue or greenish tinge. 



They move, for the most part, slowly as the word " sluggish " implies 

 though some are capable of travelling at considerable speed, especially those 

 which, like the cuttle-fish, are free swimmers. 



As to their food, some, such as the Bivalves, feed on microscopic animals 

 and debris ; others feed on plants and seaweeds ; while others are flesh-eaters. 



Mollusca are classified in two main divisions 



I. Those with a well-developed head (CEPHALOUS) and a rasping tongue. 



II. Those without a distinct head (ACEPHALOUS) and without a rasping 

 tongue. 



They are sub-divided as follows : 



Class i. Cephalopods (literally " head-footed "), in which the head has 

 eight or ten long arms, or tentacles, which are covered with suckers, and by 

 their means the creature can creep about at the bottom of the sea. These have 



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