But zooelogy has its tragedies as well as human history.  Let us turn to the development of a third molluscan line terminating in the cuttle-fishes.  The ancestors of these cephalopods, although still possessed of a shell and a high visceral hump, regained the swimming life.  First, apparently, by means of fins, and then by a simple but very effective use of a current of water, they acquired an often rapid locomotion.  The highest forms gave up the purely defensive campaign, developed a powerful beak, led a life like that of the old Norse pirates, and were for a time the rulers and terrors of the sea.  With their more rapid locomotion the supra-oesophageal ganglion reached a higher degree of development, and it was served by sense-organs of great efficiency.  They reduced the external shell, and succeeded, in the highest forms, of almost ridding themselves of this burden and encumbrance.  Traces of it remain in the squids, but transformed into an internal quill-like, supporting, not defensive, skeleton.  They have retraced the downward steps of their ancestors as far as they could.  And the high development of their supra-oesophageal ganglion and sense-organs, and their powerful jaws and arms, or tentacles, show to what good purpose they have struggled.  But the struggle was in vain, as far as the supremacy of the animal kingdom was concerned.  Their ancestors had taken a course which rendered it impossible for their descendants to reach the goal.  Their progress became ever slower.  They were entirely and hopelessly beaten by the vertebrates.  They struggled hard, but too late.

The history of mollusks is full of interest.  They show clearly how intimately nervous development is connected with the use of the locomotive organs.  The snail crept, and slightly increased its nervous system and sense-organs.  The clam almost lost them in connection with its stationary life.  The cephalopods were exceedingly active, developed, therefore, keen sense-organs and a very large and complicated supra-oesophagal ganglion, which we might almost call a brain.

The articulate series consists of two groups of animals.  The higher group includes the crabs, spiders, thousand-legs, and finally the insects, and forms the kingdom of arthropoda.  The lower members are still usually reckoned as worms, and are included under the annelids.  Of these our common earthworm is a good example, and near them belong the leeches.  But the marine annelids, of which nereis, or a clam-worm, is a good example, are more typical.  They are often quite large, a foot or even more in length.  They are composed of many, often several hundred, rings or segments.  Between these the body-wall is thin, so that the segments move easily upon each other, and thus the animal can creep or writhe.