180

ANNULOIDA.

CHAPTER XVIII.

1. GENERAL CHARACTERS OF THE ANNULOIDA.

CHARACTERS OF THE ECHINODERMATA.

2. GENERAL

SUB-KINGDOM III. ANNULOIDA (= Echinozoa, Allman).—This sub-kingdom was proposed by Professor Huxley for the reception of the two groups of the Echinodermata and the Scolecida, of which the former belonged to the old sub-kingdom Radiata, whilst the latter was formerly classed with the Annulosa. The same sections have been grouped by Professor Allman together, under the name Echinozoa; the Rotifera, however, being excluded from this division and classed with the Annulosa. By others, again, the Annuloida are looked upon as a section of the Annulosa, and not as a distinct sub-kingdom. Provisionally, however, it seems best to regard the Annuloida as one of the primary divisions of the animal kingdom, it being impossible, in the meanwhile, to frame a definition common to it and to the Annulosa. The name Vermes has sometimes been employed to designate the sub-kingdom Annuloida, certain classes being sometimes removed elsewhere, or certain others being added. In its most modern signification, the term Vermes may be held as synonymous with Annuloida, minus the Echinodermata and plus the whole of the Anarthropodous division of the Annulosa.

The Annuloida are distinguished by the presence of a distinct nervous system, and the possession of an alimentary canal which is entirely shut off from the general cavity of the body. A peculiar system of canals, usually communicating with the exterior, and termed the "water-vascular" or "aquiferous" system, is present in all; and a true vascular apparatus is sometimes present. In none is the body of the adult composed of definite segments, or provided with "bilaterally disposed successive pairs of appendages."

The union of the Echinodermata with the Scolecida in a single sub kingdom, as proposed by Huxley, must be regarded as a

purely provisional arrangement. Many other classifications have been proposed, each with some obvious advantages and some disadvantages. In some ways the most natural arrangement would be to establish a separate sub-kingdom for the Echinodermata, and to group the Scolecida with the Anarthropoda under the name of Vermes; but it seems impossible to construct a definition which will include these latter groups, and exclude all others. In the confessedly imperfect state of our knowledge, therefore, it will be as well to retain for the present the sub-kingdom Annuloida.

The Annuloida are divided into two great classes, the Echinodermata and the Scolecida.

CLASS I.-ECHINODERMATA.

Simple marine organisms, the body of the adult more or less conspicuously radiate, that of the young often distinctly bilateral. An alimentary canal, with or without a distinct anus, but never communicating with the body-cavity. The water-vascular (ambulacral) system often subserving locomotion. Nervous system radiate, composed of an esophageal ring and radiating branches. Sexes generally distinct, rarely united.

The members of this class are known commonly as Seaurchins, Starfishes, Brittle-stars, Feather-stars, Sea-lilies, Seacucumbers, &c.; and though the fully-grown animal often exhibits distinct traces of bilaterality, this is usually more or less completely masked by the general radiate arrangement of the parts of the body. On the other hand, the embryonic Echinoderm usually shows distinct bilateral symmetry. The outer layer of the general integument ("perisome") is ciliated, and the inner layer is more or less hardened by the deposition of carbonate of lime in the form of plates, granules, or spicules. In all adult Echinoderms there is a system of tubes, termed the "ambulacral system," which generally subserves locomotion, and usually communicates with the exterior. This water-vascular system surrounds the commencement of the alimentary canal, and in almost all cases gives off secondary vessels in a radiating manner. An alimentary canal is always present, and is completely shut off from the body-cavity. A vascular (pseudohæmal?) system is generally developed in addition to the true water-vessels. The nervous system in all the adult Echinoderms is a ring-like gangliated cord, which surrounds the œsophagus and sends branches parallel to the radiating ambulacral canals.

The process of development is sometimes direct; but in the

typical members of the class a characteristic form of metamorphosis occurs. The impregnated ovum gives exit to an ovoid embryo or "planula," freely locomotive by means of cilia, which are at first diffused over the body, but which soon become restricted to transverse bands, or to definite outgrowths of the body (" epaulettes "), which are disposed with bilateral

Fig. 82.-Larva of Echinus (after J. Müller). A, A, Front arms with their internal skeleton; F, F, Arms of the mouth-process; B, Posterior side-arms; a Mouth; a' (Esophagus; Stomach; & Intestine; d Ciliated bands; ƒƒ Ciliated epaulets; c Disc of the future Echinus.

symmetry. The larva or "pseudembryo" (fig. 82) next develops an alimentary canal, with a distinct mouth and anus, dividing the embryonic body into two bilaterally symmetrical halves. A mass of actively formative protoplasm now appears on one side of the stomach, within which are developed a circular and radial tubes, the whole being the rudiment of the ambulacral system of the future Echinoderm. A symmetrical calcareous skeleton, not converted into that of the adult, may be developed in the larva (as in the Echinoids and Ophiuroids), or it may be wanting (as in the Asteroids and Holothuroids). The mass of protoplasm, above mentioned as developed on one side of the stomach, rapidly increases in size, envelops the stomach, which it appropriates, and is ultimately converted into the adult Echinoderm; the remainder of the larva being absorbed or cast off as useless.

The essential peculiarity of the development of the typical Echinoderms, as above summarised, is that the larva possesses

provisional organs, which may be ultimately absorbed or thrown off, but which are not converted into the corresponding structures of the adult. Thus the larva of an Echinoid (fig. 82) possesses a mouth and alimentary canal, which are not converted into, and in no way correspond with, the mouth and alimentary canal of the adult. The larva, or "pseudembryo," as it is termed by Sir Wyville Thomson, leads a perfectly independent existence, and the true Echinoderm is developed out of a portion only of its substance.

The Echinodermata are divided into seven orders-viz., the Crinoidea, Cystoidea, Blastoidea, Ophiuroidea, Asteroidea, Echinoidea, and Holothuroidea. Of these, the first is almost extinct, and the two next are entirely so; they are really the lowest orders; but their structure will be better understood if the higher orders are considered first.

CHAPTER XIX.

ECHINOIDEA.

ORDER ECHINOIDEA.-The members of this order--commonly known as Sea-urchins-are characterised by the possession of a subglobose, discoidal, or depressed body, encased in a "test" or shell, which is composed of numerous, usually immovably connected, calcareous plates. The intestine is convoluted, and there is a distinct anus. The sexes are distinct, and the larva is pluteiform, and has a calcareous skeleton. As regards their general anatomy, the "test" of the Echinoidea is composed of numerous calcareous plates, which are generally firmly united to one another by their edges, in such a manner that the body of the animal is enclosed in an immovable box. In the singular Urchins, however, which constitute the family Echinothuride, the plates of the test overlap one another in an imbricating manner, so that the shell becomes quite flexible; and the same is the case with some of the Paleozoic Echinoids. In all living Sea-urchins, and in the great majority of the extinct forms, the test is composed of twenty meridional rows of plates, arranged in ten alternating zones (fig. 83, A), which typically pass from one pole of the shell to the other, and each of which is composed of two similar rows of plates. Five of these double rows are composed of large plates, which are

not perforated by any apertures (fig. 83, A and B, a); the zones formed by these imperforate plates being termed the

Fig. 83-Morphology of Echinoidea. A, Young specimen of Strongylocentrotus Drobachiensis, viewed from above; B, Small portion of the test of the same, magnified; C, Summit of the test of Echinus sphæra, magnified; D, Clypeaster subdepressus, viewed from above, showing the petaloid ambulacra; E, Spine of Porocidaris purpurata; F, Pedicellaria of Toxopneustes lividus. a, a Ambulacral areas; i, i Inter-ambulacral areas; g Genital plate; o Ocular plate; m Madreporiform tubercle; Membrane surrounding the anus. (Figs. A, B, and D are after A. Agassiz.)

"inter-ambulacral areas." The other five double rows of plates alternate regularly with the former, and are termed the "ambulacral areas," or "poriferous zones." Each of these zones (fig. 83, A and B, ) is composed of two rows of small plates, which are perforated by minute apertures for the emission of the "ambulacral tubes," or "tube-feet." In one great group of the Echinoids, the ambulacral areas pass from the centre of the base of the shell to its summit, when they are said to be "perfect" (ambulacra perfecta) or "simple." another great group the ambulacral, areas are not thus continuous from pole to pole, but simply form a kind of rosette upon the upper surface of the shell. In these cases-as in the