analogies are found.  In the lowest class of this division of the Animal Kingdom there is a group so similar to the Polyps, that, until recently, they have been associated with them,—­the Bryozoa.  They are very small animals, allied to the Clams by the plan of their structure, but they have a resemblance to the Polyps on account of a radiating wreath of feelers around the upper part of their body:  yet, when examined closely, this wreath is found to be incomplete; it does not, form a circle, but leaves an open space between the two ends, where they approach each other, so that it has a horseshoe outline, and partakes of the bilateral symmetry characteristic of its type and on which its own structure is based.  These series have not yet been very carefully traced, and young naturalists should turn their attention to them, and be prepared to draw the nicest distinction between analogies and true affinities among animals.

VIII.

After this digression, let us proceed to a careful examination of the natural groups of animals called Families by naturalists,—­a subject already briefly alluded to in a previous chapter.  Families are natural assemblages of animals of less extent than Orders, but, like Orders, Classes, and Branches, founded upon certain categories of structure, which are as distinct for this kind of group as for all the other divisions in the classification of the Animal Kingdom.

That we may understand the true meaning of these divisions, we must not be misled by the name given by naturalists to this kind of group.  Here, as in so many other instances, a word already familiar, and that had become, as it were, identified with the special sense in which it had been used, has been adopted by science and has received a new signification.  When naturalists speak of Families among animals, they do not allude to the progeny of a known stock, as we designate, in common parlance, the children or the descendants of known parents by the word family; they understand by Families natural groups of different kinds of animals, having no genetic relations so far as we know, but agreeing with one another closely enough to leave the impression of a more or less remote common parentage.  The difficulty here consists in determining the natural limits of such groups, and in tracing the characteristic features by which they may be defined; for individual investigators differ greatly as to the degree of resemblance existing between the members of many Families, and there is no kind of group which presents greater diversity of circumscription in the classifications of animals proposed by different naturalists than these so-called Families.