Not only is this true in a general way, but the comparison may be traced in all the details. One may ask how the narrow radiating tubes of the Acalephs, traversing the gelatinous mass of the body, can be compared to the wide radiating chambers of the Polyp; and yet nothing is more simple than to thicken the partitions in the Polyps so much as to narrow the chambers between them, till they form narrow alleys instead of wide spaces, and then we have the tubes of the Jelly-Fish. In the Jelly-Fish there is a circular tube around the margin into which all the radiating tubes open. What have we to compare with this in the Polyps? The outer edge of each partition in the Polyp is pierced by a hole near the margin. Of course when the partition is thickened, this hole, remaining open, becomes a tube; for what is a tube but an elongated hole? The comparison of the Acalephs with the Echinoderms is still easier, for they both have tubes; but in the latter the tubes are inclosed in walls of their own, instead of traversing the mass of the body, as in Acalephs, etc.
In preparing these articles on the homologies of Radiates, I have felt the difficulty of divesting my subject of the technicalities which cling to all scientific results, until they are woven into the tissue of our every-day knowledge and assume the familiar garb of our common intellectual property. When the forms of animals are as familiar to children as their A, B, C, and the intelligent study of Natural History, from the objects themselves, and not from text-books alone, is introduced into all our schools, we shall have popular names for things that can now only be approached with a certain professional stateliness on account of their technical nomenclature. The best result of such familiarity with Nature will be the recognition of an intellectual unity holding together all the various forms of life as parts of one Creative Conception.
[Footnote 3: See lower wood-cut, p. 294, _d_.]
[Footnote 4: Those who care to know more of the habits and structure of these animals will find more detailed descriptions of all the various species, illustrated by numerous plates, in the fourth volume of my _Contributions to the Natural History of the United States,_ just published.]