Evidence of a more positive kind occurs among those compound _Molluscoida_ which are most like the compound _Cœlenterata_ in their modes of union--the _Polyzoa_. Many of these form groups that are more or less irregular--spreading as films over solid surfaces, combining into seaweed-like fronds, budding out from creeping stolons, or growing up into tree-shaped societies; and besides aggregating irregularly they are irregularly placed on surfaces inclined in all directions. Merely noting that this asymmetrical distribution of the united individuals is explained by the absence of definiteness in the relations of the aggregate to incident forces, it concerns us chiefly to observe that the united individuals severally exemplify the same truth as do similarly-united individuals among the _Cœlenterata_. Averaging the members of each society, the ciliated tentacles they protrude are similarly related to prey on all sides; and therefore remain the same on all sides. This distribution of tentacles is not, however, without exception. Among the fresh-water _Polyzoa_ there are some genera, as _Plumatella_ and _Crystatella_, in which the arrangement of these parts is very decidedly bilateral. Some species of them show us such relations of the individuals to one another and to their surface of attachment, as give a clue to the modification; but in other species the meaning of this deviation from the radial type is not obvious. § 249. In the _Platyhelminthes_ good examples of the connexions between forms and forces occur. The _Planaria_ exemplifies the single bilateral symmetry which, even in very inferior forms, accompanies the habit of moving in one direction over a solid surface. Humbly organized as are these creatures and their allies the _Nemertidæ_, we see in them, just as clearly as in the highest animals, that where the movements subject the body to different forces at its two ends, different forces on its under and upper surfaces, and like forces along its two sides, there arises a corresponding form, unlike at its extremities, unlike above and below, but having its two sides alike.