302 THE MOLLUSKS. 



in those divisions. The lower forms "came tardy on." Unfamiliar and plain folks, as it 

 were, naturally less attractive ; though after a better acquaintance exhibiting many extremely 

 interesting as well as beautiful members. The sea anemones are among the most exquisite of 

 all Nature's handiwork, both in color and structure, and the numberless microscopic forms 

 now rapidly coming to light challenge our admiration and wonder. 



MOLLUSKS. 



THE. first group of invertebrated animals is called MOLLUSC A. This term is yet retained 

 for this great branch of the animal kingdom, almost meaningless though it be ; for it simply 

 expresses one, and a very unimportant, feature of this great group of animal life that of 

 softness. Probably some naturalist will stop in his work one day, and devote a little time 

 to determining the best and most comprehensive term, one which will express the nature of 

 the division as a whole. The term Arthropoda, as applied to the creatures formerly called 

 crustaceans, because they had crusty-like shells, or coverings, seems to be much nearer a 

 natural designation, as the creatures are all jointed or articulated. 



Mollusks are bilateral animals, that is, having two equal sides, though in some this is 

 obscured by certain developments, as in the gasteropods. They are either provided with a 

 bivalve or univalve shell, or none, as in the naked Mollusks. The shells are largely composed 

 of carbonate of lime, with more or less animal matter, the whole being secreted by the inclos- 

 ing mantle, from its outer layer. The shell is entirely without blood vessels. The internal 

 soft parts have a central mouth and digestive tract terminating in an anus, which is primi- 

 tively at the posterior end, and in the median line. The torsion produced by the growth in 

 the spiral and other irregular shells obscures this, but careful study of the young makes this 

 and many other seemingly difficult points clear. 



The nervous system is in the form of ganglia or knots of nerve matter with their connect- 

 ing nerve lines. The cerebral ganglia, or brain, consists of two knots above the oesophagus, 

 and pairs of knots are distributed around all the important viscera, as presiding centres of 

 nerve power. The heart, situated near what is called the back (dorsally), consists of a ven- 

 tricle and one or two auricles. It receives the blood from the respiratory organs, and forces it 

 through the body. The whole surface of the body has respiratory functions, but special 

 organs exist in the form of gills, or so-called lungs. As a rule, the sexes are united in the 

 same individual. Numerous marine gasteropods have sexes distinct, and in all of the mem- 

 bers of the highest order, the cephalopods, also. 



The power of progression in the Mollusks is greatly varied, being in some species almost 

 absent, while in others it is developed to a wonderful degree. Many of these creatures, such 

 as the mussel, the limpet, and the oyster, scarcely stir from the spot where they have once 

 fixed their habitation ; the snail and those of a similar form glide slowly along by means of 

 the curiously developed mass of muscular fibres, technically called, from its use, the foot ; the 

 scallop drives itself through the water in short jerks or nights, caused by slowly opening and 

 then rapidly shutting its valves ; several species are known to jump by a sudden stroke with 

 the foot ; the nautilus urges its shell through the waves by the violent expulsion of water 

 from its interior, and is driven along on just the same principle by which a sky-rocket soars 

 into the air ; and the flying squid, one of the cuttle-fishes, is able to rival even the flying-fish 

 in its aerial journeys, shooting throxigh the air to considerable heights, and even leaping fairly 

 over both bulwarks of a ship and alighting in the water on the opposite side of the vessel. 



The old fable of the nautilus and its sails has long been rejected, but the fabricators of 

 this legend need not have visited the ocean for an example of a molluskan boat. Any one 

 who is in the habit of watching the streamlets that irrigate while they drain our meadow 



