MOLLUSCS. 



361 



projecting a short distance outside the mantle. In Nautilus the two lobes of tlie 

 siphon are separate, being merely folded one upon the other in order to form the tube, 

 but in all other living forms this condition is character- 

 istic of an embryonic stage, and, long before the adult 

 is reached, the two halves unite. 



The shape of the mantle depends on that of the 

 body. In all it is a cup-shaped or conical envelope, 

 attached to the rest of the animal by a narrow line on 

 the dorsal surface. Its anterior margin is free, but is 

 provided with little depressions which fit over corre- 

 sponding cartilages on the siphon, so that the only 

 communication between the cavity of the mantle and 

 the exterior may be through the siphon. The mantle 

 is highly muscular and in life is constantly expanded 

 and contracted, alternately filling and emptying the 

 cavity. The water thus taken in subserves the pur- 

 poses of respiration, and also plays an important part 

 in locomotion. The animal takes the water in through 

 the free space between the neck and the mantle, and 

 then, if using it only for respiratory purposes, passes 

 it out again in the same way; but if it desires to 

 change its position, the mantle is hooked to the 

 siphonal cartilages, and then the water is forced out 

 through the siphon. Changes in direction are pro- 

 vided for by the flexibility of the end of the siphon. 

 When the animal wishes to go in a backward course, 

 the siphon is left in its normal condition ; when in a 

 forward direction, the end of the siphon is bent over 

 the edge of the mantle, so that the current is directed 

 toward the end of the body, and by the reaction loco- 

 motion is effected. 



In all but a very few of the cephalopods^ the mantle 

 secretes a shell, which may be either horny or calcare- 

 ous. In the young of all, a shell gland appears exactly 

 as in all other molluscs, but in the great majority of 

 the living forms it soon becomes enclosed by the in- 

 growth and coalescence of the edges, and hence the 

 shell is internal. In this case it is usually much longer 

 than broad, and serves to give strength and stiffness 



to the otherwise soft body. In Spir- 

 illa an exception is found. The shell 

 is enclosed in the body, but the gland 

 which forms it does not become com- 

 pletely closed. The shell, also, is not 

 a long flat object, but is tubular and 

 coiled in a spiral, and, moreover, is 

 partitioned off and traversed by a slender tube, much like that in the shell of Nauti- 

 lus now to be described. In this genus the shell is external, but the early stages 



Fig. 471. — External anatomy of Loligo 

 pealei, as shown by cutting open tne 

 mantle; a, arms; b, eye; c, siphon; tf, 

 cartilages of siphon, over winch hook 

 those of the mantle, e; /, free edge of 

 mantle; g, mantle; ft, gills; i, rectum; 

 7n, ink-bag; n^ penis; g, intestine; r, 

 veins from gills; s, gill muscles; t, 

 branchial artery; u, branchial heart; 

 V, mantle artery; w, posterior vense 

 cavse; x, visceral sac; y^ cavity of 

 mantle. 



Fig. 472. - 



■ Section of Spirula australis, showing the relations of 

 the shell to the animal. 



