This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
The communication by means of transverse canals is another arrangement exactly similar to what exists in the adult Beroeform Medusae.
Fig. 4.3. Development of Tubularia by free gemmae.
(208). The outer membrane presents eight longitudinal canals (fig. 43, g, h, b), which are found to be filled with cellules, but in which no movement has been observed. It is to the presence of these longitudinal bands that the embryo in this stage of its development owes its resemblance to certain fruits, more particularly to a melon.
(209). From the anterior part proceed four appendages (fig. 43, g, d), which were still undeveloped at the period of the detachment of the young polyp, but now insensibly unfold themselves. These are the tentacula. In the centre there projects a rounded opake body, generally of a red or yellowish tinge, which is the stomach. This viscus communicates, as has been stated above, with the four longitudinal vessels, and is the only opake part of the embryo. It opens in front by an orifice that constitutes the mouth; the whole organ is eminently contractile, turning in all directions like the body of a Hydra, sometimes elongating itself like a worm, and at others shrinking so as to be almost imperceptible. •
(210). If the embryos examined in this condition be vigorous, their movements are very varied, and the forms that they assume extremely singular. The regular contractions above noticed are the most simple actions; the two poles separate and approach each other alternately, whence results the progression of the little creature. But this contraction may be -carried to a still higher degree: the rounded stomach in the middle of the embryo not only moves itself about in every direction, but it seems to make efforts in the middle of its transparent envelope, like a worm in search of a passage by which to get out; and at length it pushes its free extremity through the opening in front of it, and elongates its body still more, until the two poles of the balloon getting approximated, the whole embryo becomes somewhat disk-shaped, or the four vessels that communicate with the stomach (if vessels they really are), by moderately contracting, form as many depressions dividing the disk into four lobes (fig. 43, h, I), or by a more forcible contraction give it the appearance of a Greek cross; and all these changes of form may take place in a few seconds.
(211). Observations are wanting relative to the manner in which the free acaleph gives origin to the fixed polyp; for although Professor Van Beneden observed the latter at a very early period after they had become attached, he was unable to witness any further changes that they undergo, and therefore gives a hypothetical outline of the forms through which he supposes them to pass, preparatory to their final establishment as young Tubulariae.
This mode of reproduction approximates the nearest to what occurs in the higher animals. Cells are observed in process of gradual organization in the middle of a vesicle, in the same manner as the vitelline cells, which are converted into an embryo. In this case the vitelline cells become aggregated and modified, so as to give rise to a new individual, which is isolated from the commencement of its existence. The point of departure for the formation of the embryo is the same as in the preceding mode of development, and the reproductive vesicle has at first precisely the same structure; but instead of preserving its transparency, this vesicle soon exhibits numerous cells, which render it more and more opake, and give it the appearance of a vitellus. In this case, moreover, there is a great difference in the relations which the red pedicle (fig. 44, a, b), bears to the embryo. In the preceding mode of development this pedicle constitutes an integrant part of the newly-formed being, forming, in fact, its stomach; but in the oviparous mode, there is no organic connexion between the one and the other, the vitellus being formed between the pedicle and the integument of the offset, and on pressing the latter between two plates of glass, these structures readily separate without any laceration.
(213). As the vitellus (fig. 44, b, a) increases in size, it becomes impacted between the integument and the pedicle, and its augmentation of bulk still increasing, the upper part of the pedicle becomes covered with it as with a hood, and at last almost entirely enveloped by it (fig. 44, c, d, e.) At this period the margins of the vitellus become indented on that side nearest the pedicle, and the tubercles between the indentations soon show themselves to be the rudiments of tentacula. The tentacula become more and more elongated, the embryo separates itself slightly from the pedicle, and a protuberance (fig. 44, F, g, 6) is then perceived in the centre of the tentacular zone, which becomes the proper body of the polyp, or rather, forms the walls of its stomachal cavity.
(214). The walls of the bud which has hitherto contained the embryo now become ruptured, and it gains its liberty (fig. 44, h.) In this condition it almost exactly resembles a young Hydra in its contracted state; and, in fact, both its body and its tentacula seem to have the same anatomical structure as those of that simply organized polyp. Having attained to this condition, its development proceeds rapidly, and it soon begins to assume the specific form of the Tubularia from which it sprung (fig. 44, I).
Fig. 44. Development of Tubularia from ova.
In the Sertularian Hydrozoa, the fleshy substance of the animal is enclosed in a ramose horny sheath, which it traverses like the pith of a tree, following all the ramifications of the branched stem of the polypary.