88 THE POPULAR SCIENCE MONTHLY. 



while the shell becomes internal instead of external. It continues to 

 be useful as a basis of muscular attachment, but no longer as a defen- 

 sive armor. 



The whole development of the mollusks, from the lowest bivalve to 

 the highest univalve form, has tended to the production of head-limbs, 

 and a compact, bag-like body. In their naked state their evolution is 

 limited by this hereditary constitution. Two modes of motion are 

 possessed, the swimming and the creeping. For use in the first there 

 is a fin-like expansion of the body, which enables them 16 move with 

 much rapidity, while backward motion is gained by expulsion of water 

 from between the arm-membranes. But the body continues rigid, and 

 is at a disadvantage as compared with the flexible worm type. 



Creeping motion is gained by a development of sucking-disks upon 

 the arms, which serve for a slow dragging of the body, turned head 

 downward, and also as an efficient agent in the capture of game. 



This highest moUusk, the cuttle-fish, is utterly unfitted for a land 

 residence despite its acute sense-organs. The ink-bag, which ena- 

 bles it to conceal itself in the water, would be of no use to it on land ; 

 its tail-fins or its radiated head-arms could not be changed into effi- 

 cient organs of land-motion ; it would, therefore, be at a great disad- 

 vantage as compared with the body-limbed, flexible-framed vertebrates^ 

 Thus the highest development of the mollusk type is unsuited by its 

 defective constitution to a land residence, and can only progress to 

 the limited extent permitted by the restrictions of a water residence. 



In the Echinoderms a similar lengthening of the body is gained. 

 Of the free forms, we have the flattened starfish, with the arms some- 

 times developed at the expense of the body, the body sometimes at the 

 expense of the arms ; the globular sea-urchin, with its ambulacral 

 arms ; and the lengthened Holothuroid. In this latter is displayed what 

 seems almost an intelligent effort to imitate the worm type. Unlike 

 the other Echinoderms, its intestinal axis becomes horizontal instead of 

 vertical. Thus, like the worms, it attains dorsal and ventral surfaces, 

 exposed to diverse conditions. As a consequence, of its five rows of 

 ambulacral suckers, those on the dorsal surface disappear in the most 

 advanced genera, only the three ventral rows being retained. The 

 distinguishing radiate structure is displayed only by its circle of mouth- 

 tentacles, the food-getting organs. It also loses the calcareous outer 

 armor of the lower Echinoderms, replacing it by a flexible, leathery 

 skin. 



But, with these several advances toward the worm type, the heredi- 

 tary disadvantages of the Holothuroid act as impassable restrictions to 

 any great development. The organs of the higher senses are wanting. 

 It is in no way adapted to swimming, its exterior organs being quite 

 unfitted to develop into fins. Nor are the ambulacral suckers suited 

 to any rapid progression. An utter change in character would be 

 necessary to adapt them to a walking or running movement. Thus 



