SERTULARIDAE In the Sertularian Hydrozoa, the fleshy substance of the ani mal is inclosed in a ramose, horny sheath, which it traverses like the pith of a tree, following all of the ramifications of the branched stem or polypary. The general stem of the polypary is entirely filled with a fleshy substance, exactly resembling, in its nature, the tissue composing the body of the polyp, whereby all the indi viduals belonging to the common stock are brought into com munication with each other. " The Hydroida excel all other Zoophytical productions in delicacy, and the graceful arrange ment of their forms, some borrowing the forms of the prettiest marine plants, others assuming the semblance of the ostrich plume, while the variety and elegance exhibited in the figures and sculpture of their miniature cups and chalices is only limited by the number of their species." In the Sertularidce, whole compound animal is inclosed in a tube of transparent substance, somewhat flexible, though firm, resembling horn, an exudation from the gelatinous integu ment, and this tube, at every bud, takes the form of an open cell or cup, (varying much in shape, according to the species,) into the cavity of which each individual polyp head can withdraw itself on alarm, and from the orifice of which it protrudes and ex pands when seeking prey.
These little Polyps provide nourishment for the whole. On examining a piece of one of these polyparies with a good glass, the tubular horny envelope is seen to be filled with granular matter, and on attentively watching it, globules will be seen moving in different directions, producing a sort of circulation, or cyclosis, very much resembling what is observable in some plants. The globules thus moving, do not appear to be con tained in vessels, but steal in slow currents, ascending along the sides and returning down the middle in an opposite direction.
It is interesting to view these Polyps with a microscope : " Protruding themselves beyond the mouths of their cells, they inflect their bodies in all directions in quest of prey, waiting until some passing body impinges upon their tentaeula, which is at once seized and conveyed into the stomach with a rapidity and dexterity almost beyond belief. The multiplication of these singular animals appears to take place in three different modes : first, by cuttings, as in plants ; second, by offshoots, or the forma tion of new branches bearing Polyps ; and, third, by gemmules capable of locomotion. The first mode strikingly resembles what is observed in the vegetable kingdom ; for, as every branch of the plant-like body contains all the parts necessary to independ ent existence, it can hardly be a matter of surprise that any por tion, separated from the rest, will continue to grow and perform all the functions of the entire animal.
" The second mode of increase, namely, by the formation of new branches and Polyps, seems more like the growth of a plant than the development of an animal. We will consider it under two points of view : first, as regards the elongation of the stem ; secondly, as relates to the formation of fresh cells containing the nutritive Polyps. On examining any growing branch, it will be found to be soft and open at the extremity, and through the ter minal orifice, the soft tegumentary membrane above described, as forming the tube, is seen to protrude ; the skeleton is not, there fore, merely secreted by the inclosed living granular matter, but it is the investing membrane, which continually shoots upwards and deposits hard material in its substance, as it assumes the form, and spreads into the ramifications peculiar to its species.
" Having thus lengthened the stem to a certain distance, the next step is the formation of a cell and a new Polyp, which is accomplished in the following manner : The newly formed branch has at first precisely the appearance and structure of the rest of the stalk of the Zoophyte, being filled with granular matter, and exhibiting in its interior the circulation of globules already de scribed, moving towards the extremity along the sides of the tube, and in an opposite course in the middle ; the end of the branch, however, before soft and rounded, soon becomes perceptibly di lated. After a few hours, the branch is visibly longer, its extre
mity more swollen, and the living pith is seen partially to have separated itself from the sides of the tube, the boundaries of which become more defined and undulating. The growth still proceed ing, the extremity is distinctly dilated into a cell, in which the soft substance seems to be swollen out, so as to give a rude out line to the bell-shaped Polyp, but no tentaculm are yet distin guishable. A rudimentary septum is now visible, stretching across the bottom of the cell, through the centre of which the granular matter, now collected into a mass occupying but a portion of the stem, is seen to pass. The Polyp and cell gradually grow more distinct, and the tentaculie become distinguishable. As the de velopment proceeds, the tentaculee become more perfect, and the Polyp at length rises from its cell to exercise the functions for which it is destined. The third mode of multiplication, or that by reproductive gemmules, seems to be especially adapted to the diffusion of the species, and is peculiar to Zoophytes of this de scription. At certain periods of the year, besides the ordinary cells which contain nutritive Polyps, others are developed from different parts of the stems, which may be called female or fertile Polyps, although usually simply termed the vesicles. The cells of this kind are much larger than the nutritive cells, and of very different forms; they are, moreover, deciduous, falling off after the fulfilment of the office for which they are provided. They are produced in the same manner as the rest of the stem, by an extension of the tegumentary membrane, which, as it expands into the form of the cell, becomes of a horny texture. It may be traced, however, over the opening of the cavity, where it some times forms a movable operculum." Now, within this tall, transparent, vase-like capsule, by a series of gradual changes, the common nutrient flesh develops a very peculiar embryo. "When somewhat mature, the permeating tube is seen swollen out into separate ovate sacks, ten or more in num ber, each of which contains several embryos. Those nearest the mouth of the vesicle are first developed, and escape successively by slowly emerging from the pitcher-like rim." This little medusa-like creature, freed from its parent cell, has the power of moving about rapidly in all directions through the water.
Mr. Peach, who first observed them, says :—" They resemble umbrellas without handles, or very wide and short hand bells." He also remarks :—" They assume various positions, and, when in the water, remind me of thousands of parachutes thrown from a balloon, descending in various states of expansion." The phe nomena of which we have spoken, strikingly illustrate the law known as " the alternation of generations," and may be briefly summed up as follows The Polype, a fixed and rooted ani mal, increases its own individual life for awhile, by putting forth a succession of budding heads, but at a certain period gives birth to a number of beings that bear no resemblance to itself in form or habit, but are, to all intents and purposes, free, swimming Each of these, after pursuing its giddy course for a time, produces a number of eggs, which change into active ani mals, having the closest resemblance to infusoria. Each of these latter presently becomes stationary, and affixed to some foreign body, along which it creeps as a root thread, shooting up tubular and celled Polypes.
" Hence, any one individual is not at all like its mother or its daughter, but exactly resembles its grandmother or its grand daughter." In other words, the alternations are as follows : 1. The medusa produces eggs. 2. The eggs, after passing through an infusorial state, become Polypes, like Corynm, Tubularice, or Campanulariw. 3. The Polype produces a kind of bud that finally drops off and becomes a medusa. Thus, the egg of a Me dusa, in such cases, does not produce a Medusa except after going through the intermediate state of the Polype. Or, if we com mence with the Polype, the series is : 1. The Polype produces bulbs that become Medusre. 2. The Medusae produce eggs. 3. The eggs produce Polypes.