The outward appearance of sponges has an exceptionally wide range, so that it is difficult to give a novice any very definite picture of what he is to expect when searching for these animals. This diversity is in part due to the absence of organs of sufficient size to determine the shape of the whole or limit its variation, {166}that is to say, the separate organs are of an order of size inferior to that of the entire body. The animals are fixed or lie loose on the sea bottom; there are in no case organs of locomotion, and again no sense-organs, no segregated organs of sex, and as a rule no distinction into axis and lateral members. It is by these negative characters that the collector may easily recognise a sponge. HISTORY.—Sponges are, then, in many of their characters unique; and they present a variety of problems for solution, both of special and general interest, they are widely distributed in time and space, and they include a host of forms. It therefore causes no little surprise to learn that they have suffered from a long neglect, even their animal nature having been but recently established. Though known to naturalists from the time of Aristotle, sponges have been left for modern workers as a heritage of virgin soil: it has yielded to them a rich harvest, and is as yet far from exhausted. The familiar bath sponge was naturally the earliest known member of the phylum. It is dignified by mention in the _Iliad_ and in the _Odyssey_, and Homer, in his choice of the adjective "full of holes," πολύτρητος, shows at least as much observation as many a naturalist of the sixteenth and seventeenth centuries. Aristotle based his ideas of sponges entirely upon the characters of the bath sponge and its near allies, for these were the only kinds he knew. With his usual perspicuity he reached the conclusion that sponges are animals, though showing points of likeness to plants. The accounts of sponges after Aristotle present little of scientific interest until the last century. Doubtless this is in part due to the absence of organs which would admit of dissection, and the consequent necessity of finer methods of study. Like other attached forms, sponges were plant or animal as it pleased the imagination of the writer, and sometimes they were "plant animals" or Zoophyta: those who thought them animal were frequently divided among themselves as to whether they were "polypous" or "apolypous." An opinion which it is somewhat difficult to classify was that of Dr. Nehemiah Grew,[186] who says: "No _Sponge_ hath any Lignous Fibers, but is wholly composed of those which make the Pith and all the pithy parts {167}of a Plant, ... So that a _Sponge_, instead of being a _Zoophyton_, is but the one-half of a Plant."