while the shell becomes internal instead of external. It continues to be useful as a basis of muscular attachment, but no longer as a defensive 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 mollusk, the cuttle-fish, is utterly unfitted for a land residence despite its acute sense-organs. The ink-bag, which enables 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 efficient organs of land-motion; it would, therefore, be at a great disadvantage 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 sometimes 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 hereditary 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