Characteristics of the Polyzoa—Details of structure according to Allman—Plumatella repens—Its great beauty under proper illumination—Its tentacles and their cilia—The mouth and its guard or epistome—Intestinal tube—How it swallowed a Rotifer, and what happened—Curiosities of digestion—Are the tentacles capable of Stinging?—Resting Eggs, or "Statoblasts"—Tube of Plumatella—Its muscular Fibres—Physiological importance of their structure.
URING the fag end of last month I observed some fragments of a new creature among some bits of Anacharis, from the Vale of Heath Pond, and searched for complete and intelligible specimens without effect. Luckily one evening a scientific neighbour, to whom I had given some of the plant for the sake of the beautiful Stephanoceri which inhabited it, came in with a glass trough containing a little branch, to which adhered a dirty parchment-like ramifying tube, dotted here and there with brown oval masses, and having sundry open extremities, from which some polyp-shaped animals put forth long pearly tentacles margined with vibrating cilia, and making a lively current. The creatures presented an organization higher than that of polyps, for there was an evident differentiation and complication of parts. They belonged to the Polyzoa or Bryozoa,[21] a very important division of the mollusca. The Polyzoa are chiefly marine, and the common "sea-mat," often erroneously treated as a sea-weed, is a well-known form. A species of another order often picked up on our coasts is the Sertularia, or Sea-Fir, composed of delicate branching stems of a horny-looking substance, which, under a pocket-lens, is found to contain an immense number of small cells inhabited by Polyps. It is instructive to compare the two and note how much more advanced in structure is the Polyzoon than the polyp.
[21] Polyzoa means "many animals," in allusion to their habit of living in association. Bryozoa, "moss-animals," from some forming cells having that appearance.
Plumatella repens. Single Polypide enlarged
Polyzoa were formerly associated with the polyps, to which they bear a strong superficial resemblance; but they are of a much higher degree of organization, as will be seen by comparing what has been said in a former chapter on the Hydra, with the description which we now proceed to abridge from Dr. Allman's splendid monograph on the fresh-water kinds. In order to get a general conception of a Polyzoon, the Professor tells us to imagine an alimentary canal, consisting of œsophagus, stomach, and intestine, to be furnished at its origin with long ciliated tentacles, and to have a single nervous ganglion on one side of the œsophagus. We must then conceive the intestine bent back till its anal orifice comes near the mouth; and this curved digestive tube to be suspended in a bag containing fluid, and having two openings, one for the mouth and the other for the vent. A system of muscles enables the alimentary tube to be retracted or protruded, the former process pulling the bag in, and the latter letting it out. The mouth of the bag is, so to speak, tied round the creature's neck just below the tentacles, which are the only portions of it that are left free. The investing sack has in nearly every case the power of secreting an external sheath, more or less solid, and which branches forming numerous cells, in which the members of the family live in a socialistic community, having, as it were, two lives, one individual, and the other shared in common with the rest.
The whole group of tubes and cells, whatever may be the form in which they are aggregated, is called the Polypary, or, as Dr. Allman prefers, the Cœnœcium (common house); the creature he names a Polypide[22] (polyp-like); and the disk which bears the tentacles Lophophore (crest-bearer). There are some more hard words to be learnt before the student can enjoy himself scientifically among the Polyzoa, and we shall be compelled to employ some of them before we have done; but will now endeavour to describe what was presented to our view by the specimen obtained from the Hampstead Pond.
[22] Polyzoon is preferable, as avoiding confusion with polypite, used for another class of object.
The general aspect of a branch of Plumatella repens—the creature we have to describe—is given in the drawing annexed. When all was quiet, the mouths of the bags belonging to each cell were slowly everted, and out came a numerous bundle of tentacles, which were either spread like the corolla of a flower, or permitted to hang dishevelled like the snake-locks of Medusa. We will suppose these organs symmetrically expanded, and that we are looking down upon them with a magnifying power of sixty diameters, the light having been carefully adjusted by turning the reflecting mirror a little on one side, to avoid a direct glare. The tentacles, each of which curves with a living grace, and displays an opaline tint in its glassy structure, do not form a complete circle, for at one place we discern two slightly diverging arms of the disk, or frame (Lophophore) from which they grow.