These are usually known by the names of ovigerous vesicles. (Fig. 61, e.) The late Professor E. Forbes has demonstrated that these vesicles are not new organs differing in their nature from other parts of the organism, but that they are really modifications of a part or parts for a special purpose. "The vesicle is formed from a branch or pinna, through an arrest of individual development by a shortening of the spiral axis, and by a transformation of the stomachs (individuals) into an ovigerous placenta, the dermato-skeletons (or cells) uniting to form a projecting capsule or germen, which metamorphosis is exactly comparable with that which occurs in the reproductive organs of flowering plants, in which the flower-bud (normally a branch clothed with spirally arranged leaves) is constituted through the contraction of the axis, and the whorling of the (individual) appendages borne on that axis, and by their transformation into the several parts of the flower (reproductive organisms.)"*

The vesicles are, therefore, branches modified for a special purpose in the economy of the animal, in the same way as we have seen that the parts of the flower in plants are merely modifications of the typical appendage (the leaf) arranged upon a shortened axis. In some instances special buds or individuals issue from the vesicles, becoming detached to enjoy for a time an independent existence, for which they are accordingly fitted by special

FIG. 62.*

FIG. 62. Medusoid bud (See Fig. 61, e) of Campanularia; it swims freely in the water. a, body; b, mouth; c, upper surface; d, cirri. It is believed to produce ova, which are developed into a Campanularia stock.

modification. Through means of these buds the true reproductive process by ova is effected. These individuals when fully matured are flat discs, they may be compared to an expanded umbrella with a short stalk. The margin of the disc is provided with appendages, by which, and by its own contractile powers, the organisms move and disperse themselves in the water. They, in fact, correspond to the flower in plants. Their organization fits them admirably for an independent existence, and for dispersing the ova at a distance from the parent stock.

The sea-anemones may in general terms be compared to a jointless cylin

der, the extremities

of which present two distinct modifications in accordance with their function. The base or

lower end is that by

which the animal

is fixed to a shell or

rock, the part acting on the principle of the sucker, but capable also of becoming detached and

performing lateral progression to a new place at the will of the animal. The free end of the body presents one or more radiate series of hollow tentacula capable of protrusion or retraction, and this by a very simple mechanism, the injection or expulsion of water. When fully ex

FIG. 63. To show form and structure of Actinia or sea-anemone. a, point of attachment or base; b, mouth, c, tentcula; e, stomach; gk, partitions or vertical plates; h, passages into tentacula.

panded, these tentacles are effectual means of capturing and retaining prey, and of conveying it to the central mouth. This opening leads by a short canal to the capacious stomach, the outer surface of which is connected with the walls of the body by a number of radiating vertical plates. The cells formed by the plates, which are muscular, have a special function as regards the protrusion of the tentacula. As there is a certain order in the arrangement of these organs, we find corresponding distribution of the vertical plates.

In our native species of Helianthoida, reproduction by ova is the most usual mode; reproduction by buds is less common. But in many of the varied and beautiful stone corals of tropical seas, colonies are formed by the budding process. And here we meet with interesting modifications in harmony with this mode of increase, and the localities where the animals usually occur. A colony of soft Actinias could not attain any great size, and at the same time resist the destructive influence of a turbulent ocean. The species of Millepora, Madrepora, etc., so well known to navigators, are Helianthoida, which have the peculiar power of separating carbonate of lime from the sea-water, and building it up in forms which equally astonish us by their size, and please us by the beauty of their details. The coral-builders, it is well known, thrive best in the surf of the breakers, and their peculiarities of organization fit them admirably for such localities. In many of them we find calcareous matter deposited in the interstices of the perpendicular plates already alluded to, which afford support to the soft parts, and enable them to resist the action of the surrounding medium; the sea-water, at the same time, yields to them the material for such purpose. In former epochs of the earth's history, as well as in our own, the coral-builders

have contributed in no small degree to modify the earth's surface, and prepare it for the abode of higher animals.

The statements already made regarding the compound Hydroida and their detached animal-flowers, apply also to certain of the Acalepha or sea-jellies.

The true Medusæ commence

their existence as animals resembling in no small degree the common fresh-water polyp. They multiply for a time, by a process of budding, and the final effort is to produce other buds which become developed into the fullgrown Medusa.

FIG. 64*

In both the conditions the radiate type is retained, but in each kind of organism there is a

special modification in accordance with the mode of life. The ordinary buds are modified in accordance with their sedentary existence; one end forms a point of attachment, the other is provided with tentacula for the capture of food. The other special buds, which pass off from the common stock, are fitted for independent existence, and for progression in the water; they move from place to place by the undulations of the umbrella-shaped disc and the action

FIG. 65.t

FIG. 64. Original stock, or polype condition of Medusa. Shews a group of five, four of which have sprouted as buds from the original stock.

FIG. 65. Polyp of Medusa, producing young Medusæ. a, the stock or body; e, a bud, as in last figure; c, tentacula of stock; d, young Medusa, (corresponds to flower-bud in plants,) with its tentacula and proboscis. Tentacula, c, are a second growth.

of marginal appendages. They are provided with a digestive system, and organs for capturing prey, and, finally,

B

A

FIG. 66.*

produce abundance of ova, each of which becoming fixed to a rock or shell forms a polyp stock, and gives origin to similarly modified organisms.

Among Echinoderms, as we have already seen, there is remarkable unity amidst great diversity of form and consistence of parts. This diversity in particular cases has an evident relation to the wellbeing of the species. The hard covering of starfishes constitutes a mailed defence, combining, in most instances, strength and flexibility. The many pieces (thousands) of which it consists in some species, are evidently suited for both the functions mentioned. While the ovarian plates, pierced for the passage of the ova, and the ambulacral, giving exit and support to the delicate cirri, respectively occupy important relations as regards the economy of the animal.

The Comatula, or rosy star of our own seas, presents modifications in conformity with its habits. In its adult condition it can cling to a rock, a sea-weed, or a coral, by means of the simple-jointed arms with which it is provided for that purpose; while, on the other hand, the large pinnated arms may be used for free progression in

FIG. 66. Advanced state (Medusa) of d, Fig. 65. A, side view; a, proboscis; b, lobes, or subdivisions of margin. B, upper view of A, shows quadrilateral mouth in the centre.