STRUCTURE OF RADIATA.

-ECHINODERMATA.

125

117. The circular arrangement of the organs of Radiated animals is a striking point of resemblance to the Vegetable kingdom; and it has frequently caused mistakes to be made in regard to the Sea-Anemones and other large polypes, which, when their mouths are open and their arms spread out, look so much like the blossoms of some of the Composite tribe of plants, as to have received the name of animal flowers. But there is yet a stronger analogy between the lower members of the Radiated group and the Vegetable kingdom; for among the former, as in the latter, we find a union of many individuals, which are capable of existing separately, into one compound structure, having a plant-like form. This is the nature of the stem of Coral (fig. 76); which is, in fact, the skeleton of one of these compound systems, consisting of a number of polypes united by a jellylike flesh; just as the woody stem of a tree is the skeleton that supports a vast number of buds, each of which is capable of living by itself. This aggregation results from the indefinite multiplication of parts by the process of gemmation or budding, and from the persistence of the connexion between these parts, notwithstanding that, if separated, they can maintain an independent existence. To the treelike fabrics thus produced, the name Zoophytes (animal plants) is commonly given; and ordinary observers often find it difficult to get rid of the idea of their vegetable origin. The animals that formed them are, of course, fixed to one spot during all but the earliest periods of life; and the amount of movement which they perform, for the purpose of obtaining and securing their food, is very little greater than that which is witnessed in the Sensitive plant and Venus's fly-trap.

118. The class of ECHINODERMATA receives its name from the prickly character of its covering, which is evident enough in the Echinus or Sea- Urchin, and in the Star-fish; but there are other animals, sufficiently resembling these in general structure to be united in the same class, which have a body entirely soft, namely, the Holothuria (fig. 67), commonly termed Sea-Cucumbers. This class ranks as the highest among the Radiata, in regard to general complexity of structure. The skeleton of the Sea-Urchin, Star-fish, and other animals resembling them, is a box-like shell or "test," formed

126

STRUCTURE OF ECHINODERMATA.

of a great number of pieces, very regularly arranged and united together (fig. 69, e); but these pieces are for the most

part only repetitions of one another; and the different portions have not that variety of uses which we see in higher

animals. With the exception of the tribe of Encrinites or lily-like animals (fig. 68), of which there are very few now existing, but which were very abundant in former ages, all the animals belonging to this class are unattached, and are capable of moving freely from place to place. Their motions are very sluggish, however, and are principally effected by means of an immense number of minute tubular feet (fig. 68, c), furnished with suckers at their extremities, which can be projected from almost any part of the body. These are seen in rows on the under side of each arm of the Star-fish; they are put forth through rows of very minute apertures in the shell of the SeaUrchin (commonly termed the Sea

STRUCTURE OF ECHINODERMATA.

127

Egg); and they are also arranged in rows on the surface of the body of the Holothuria, as seen in fig. 67.

119. The radiated arrangement is very evident in the whole bodies of the Star-Fish (fig. 65), and Echinus or SeaUrchin (fig. 69); but in the Holothuria (fig. 67) it is nearly confined to the parts about the mouth; which, however, exhibit it so completely, that such an animal cannot be mistaken for one of the Articulated series, even though, as sometimes happens, the body is prolonged into a worm-like form. The digestive apparatus in this class has usually a high degree of complexity, as will be seen by the accompany

ing figure (fig. 69), which shows the interior of an Echinus, whose globular shell has been sawn across its equator, so as

128

STRUCTURE OF ECHINODERMATA AND ACALEPHÆ.

The mouth, k,

to allow of the separation of its two halves. situated at one of the poles of the shell, is surrounded by a very curious apparatus of jaws and teeth (fig. 69), which forms what is termed the "lantern;" from the mouth commences the long narrow œsophagus, m, that leads to the stomach, n, which is merely a dilated portion of the alimentary tube; the continuation of this, o, q, r, forms the intestinal canal, which winds once round the shell, and then doubles back and winds in the opposite direction, terminating at the anal orifice, s, which is situated at the opposite pole. The intestine is held in its place by a double fold of the membrane lining the shell, resembling the mesentery of higher animals; the blood is distributed over this membrane, to be exposed to the aerating influence of the water admitted into the cavity of the shell; and the water is kept in movement by the cilia with which the membrane is clothed. Round the anus, s, are seen the five branching ovaries, each of which discharges its contents by a distinct orifice. The circulating apparatus is imperfect, the blood not being impelled by a distinct heart; still, however, it moves in great part of its course through proper vessels, and not through mere channels in the tissue. In the Star-fish, however, the body is very much flattened; and the stomach, instead of having a separate intestinal tube with a distinct orifice, is a mere bag with a single aperture, which serves both to take in food and to cast forth the indigestible remains. This character will be found to prevail among all the inferior Radiata.

120. The radiated structure is also well seen in the greater number of animals forming the group of ACALEPHÆ, or Sec= Nettles. Their bodies are entirely soft and jelly-like; containing so small a quantity of solid matter, that, when upon being taken out of the water their fluid drains away, there is scarcely anything left; hence they are commonly termed Jelly-Fish. They derive their other name of Sea-Nettles from the stinging power which most of them possess. They are formed to float freely in the water; but they do not in general possess any means of actively propelling themselves through it. The radiated arrangement is very regularly pre. served in some of this group, whilst it is less evident in others. The accompanying figure (fig. 70) represents one of the Medusa tribe, as seen floating in water. The umbrella

STRUCTURE OF ACALEPHE.

129

shaped disc above contains the stomach, which is placed in the centre, and which opens by a single orifice or mouth, directed downwards. Around the stomach are four chambers, in which the eggs are prepared. The mouth is surrounded by four large tentacula, which bring to it the necessary supply of

food; and other tentacula are seen, in this species, to be hanging from the edge of the disc. In the edge of this disc, the nutritious fluid, which flows in channels prolonged from the stomach and excavated out of the soft tissues, seems to be exposed to the influence of the surrounding water; but nothing like a heart or a regular circulation exists.-Recent discoveries in regard to the developmental history of the Medusa and their allies, have rendered it very doubtful whether the Acalepho should continue to take rank as a distinct class; since many of them constitute only a particular phase in the life of the Hydroid Zoophytes (§ 124).

121. The class of POLYPIFERA, or coral-forming animals, commonly known as Zoophytes, includes two principal tribes, which differ from one another in structure to such a degree as to

K