At the extremity of each of the four longi tudinal vessels a little tubercle now becomes developed, which, as it becomes elongated, is converted into a tentacle, or sometimes, as in Eudendriunz, by its bifurcation, two tentacula are formed from each tubercle.
At this period of its development the young Tubularia spontaneously' detaches itself from the parent stem, presenting at the moment of its separation the appearance of a balloon, or rather of a melon. (Ftg A, 2, 3.) Itscontrae tions become more and more lively, and it is by the aid of these movements that its sepa ration is effected. The two poles of its globular body may be seen to approach each other, and to separate alternately, with a movement of systole and diastole similar to what is observable in many Aledusm. No traces of cilia are observable either externally or in the interior of its body. In this con dition it presents an external covering, which is, so to speak, merely a derivation from the integument of the parent polype : this cover ing presents somewhat more consistence than the internal parts, and is open in front.
A second membrane lines the preceding throughout its whole extent ; like the former, it is quite transparent, and at the anterior opening (e) is prolonged internally to a little distance, forming a sort of funnel. These walls enclose four vessels (c), which extend from the base of the embryo and open in front into the hollow zone (h), frora which the tentacula take their origin. These longitudinal vessels therefore communicate with each other by a transverse canal, and at their origin open into the central or digestive cavity, which will, presently, be more particularly de scribed. From this disposition it results that the contents of the stoinach can pass as far as the extremities of these four vessels, and by means of the transverse canal can be trans ferred from one to the other. Professor Van Beneden observed a fluid containing globules moving in this direction in their interior. The communication between the longitudinal ves sels and the stomachal cavity, and their inter communication by means of transverse canals, is another arrangement exactly similar to what exists in the adult Medusas.
The outer membrane presents eight longi tudinal canals, hich 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.
From the anterior part proceed four ap pendages (d), which were still undeveloped at the period of the detachment of the young polype, but which insensibly unfold them selves. These are the tentacula. hi the centre there projects a rounded opaque body (b), generally of a red or yellowish tinge, which is the stomach. This viscus communicates, as has been stated above, with the four lon gitudinal vessels, and is the only opaque part of the embryo. It opens in front by an orifice which constitutes the mouth ; the whole organ is eminently contractile, turning itself in all directions like the body of a Hydra, sometimes elongating itself like a worm, and at others shrinking so as to be almost imper ceptible.
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 alter nately, 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 contracts itself in every direction, but it seems to turn itself 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 becoming ap proximated, the whole embryo becomes some what disc-shaped, or the four vessels that communicate with the stomach (if vessels they really are), by moderately contracting, form as many depressions dividing the embryo into four lobes (fig.50. 5, 6.), 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.