The Principles of Biology, Volume 2 (of 2)

by Herbert Spencer

The members of those compound Cœlenterata which move through the water by their own actions, in attitudes that are approximately constant, show us a more or less distinct two-sidedness. Diphyes, Fig. [259], furnishes an example. Each of the largely-developed and modified polypites forming its swimming sacs is bilateral, in correspondence with the bilateralness of its conditions; and in each of the appended polypites the insertion of the solitary tentacle produces a kindred divergence from the primitive radial type. The aggregate, too, which here very much subordinates its members, exhibits the same conformity of structure to circumstances. It admits of symmetrical bisection by a plane passing through its two contractile sacs, or nectocalyces, but not by any other plane; and the plane which thus symmetrically bisects it, is the vertical plane on the two sides of which its parts are similarly conditioned as it propels itself through the water.

Fig. 259.

Another group of the oceanic Hydrozoa, the Physophoridæ, furnishes interesting evidence—not so much in respect of the forms of the united individuals, which we may pass over, as in respect of the forms of the aggregates. Some of these are without swimming organs, and have their parts suspended from air-vessels which habitually float on the surface of the water. Hence the distribution of their parts is asymmetrical. The Physalia, Fig. [152], is an example. Here the relations of the integrated group of individuals to the environment are indefinite; and there is thus no agency tending to change that comparatively irregular mode of growth which is probably derived from a primordial type of the branched Hydrozoa.

Fig. 152.

So various are the modes of union among the compound Cœlenterata, that it is out of the question to deal with them all. Even did space permit, it would be impracticable for any one but a professed naturalist, to trace throughout this group the relations between shapes and conditions of existence. The above must be taken simply as a few of the most significant and easily-interpretable cases.

§ 248. In the sub-kingdoms Polyzoa and Tunicata we meet with examples not wholly unlike the foregoing. Among the types assembled under these names there are simple individuals or aggregates of the second order, and societies or tertiary aggregates produced by their union. The relations of forms to forces have to be traced in both.

Solitary Ascidians, fixed or floating, carry on an inactive and indefinite converse with the actions in the environment. Without power to move about vivaciously, and unable to catch any prey but that contained in the currents of water they absorb and expel, these creatures are not exposed to sets of forces which are equal on two or more sides; and their shapes consequently remain vague. Though internally their parts have a partially-symmetrical arrangement, due to their derivation, yet they are substantially unsymmetrical in that part of the body which is concerned with the environment. Fig. [156] is an example.[45] Among the composite Ascidians, floating and fixed, the shape of the aggregate, partly determined by the habitual mode of gemmation and partly by the surrounding conditions in each case, is in great measure indefinite. We can say no more about it than that it is not obviously at variance with the laws alleged.

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.