ANNULOIDA.

CHAPTER XVIII.

1. GENERAL CHARACTERS OF THE ANNULOIDA. 2. GENERAL CHARACTERS OF THE ECHINODERMATA.

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 subkingdom. 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 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.'

By Professor Allman the following definition has been given of the Annuloida or Echinozoa ::

'Animals composed of numerous merosomes' (body-segments) 'which are disposed radially round a longitudinal axis; always with a determinable, median, antero-posterior and dorso-ventral plane. A distinct body-cavity from which the alimentary canal is entirely shut off, and which, therefore, never communicates with the outer world through the mouth.'

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

CLASS I. ECHINODERMATA.

The members of this class are known commonly as Seaurchins, Star-fishes, Brittle-stars, Feather-stars, Sea-cucumbers, &c., and the following are their leading characteristics. They are all animals which, in the adult condition, show a more or less distinctly radiate condition of their parts, especially of those around the mouth; whilst in their embryonic stages they are more or less distinctly bilaterally symmetrical. The external envelope of the body ('perisome') is either composed of numerous calcareous plates, articulated together, or of a coriaceous integument, in which calcareous granules and spicules are usually developed. In all adult Echinoderms there is a system of tubes, termed the 'ambulacral system,' which subserves locomotion, and usually communicates with the exterior. An alimentary canal is always present, and is completely shut off from the body-cavity. In many, if not in all, both neural and hæmal systems are developed. The nervous system in all the adult Echinoderms is a ring-like gangliated cord, which surrounds the oesophagus, and sends branches parallel to the radiating ambulacral canals.

The special features of the structure of the Echinodermata will be noticed under each order, but it will be as well to give here an abstract of Professor Huxley's description of the process of development in the members of the class. In the great majority, if not in all, of the Echinodermata the impregnated ovum is developed into a free-swimming, ciliated, ovoid embryo. Soon the cilia become restricted to one, two, or more bands, which are generally disposed transversely to the long axis of the body, and are in all cases bilaterally symmetrical. The parts of the body which support the cilia are usually developed into protuberances, or processes, which are symmetrically disposed upon the two sides of the body. The larvæ of Asteridea and Holothuridea are devoid of any continuous skeleton, but those of Ophiuridea and Echinidea possess a very remarkable, bilaterally symmetrical, continuous, calcareous skeleton, which extends into, and supports the processes of the body.' In this stage the larva form of the two orders last mentioned was described by Müller as a distinct animal, under the name of Pluteus, from its resemblance to a painter's easel. (See fig. 33, 1.)

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An alimentary canal soon appears in the larva, forming a curve with an open angle towards the ventral surface of the organism. The parts of the alimentary canal consist of a mouth, gullet, globular stomach, and short intestine, with a distinct

anal aperture; the whole being disposed in a longitudinal and vertical plane, dividing the larval body into two symmetrical halves.' Besides the digestive canal, no other organs have hitherto been discovered in these larvæ. In the further process of development, an involution of the integument takes place upon one side of the dorsal region of the body, so as to give rise to a cæcal tube, which gradually elongates inwards, and eventually reaches a mass of formative matter, or blastema, aggregated upon one side of the stomach. Within this, the end of the tube becomes converted into a circular vessel, from which trunks pass off, radially, through the enlarging blastema. The latter, gradually expanding, gives rise in the Echinidea, the Asteridea, the Ophiuridea, and the Crinoidea to the body-wall of the adult; the larval body and skeleton (when the latter exists), with more or less of the primitive intestine, being either cast off as a whole, or disappearing, or becoming incorporated with the secondary development, while a new mouth is developed in the centre of the ring formed by the circular vessel. The vessels which radiate from the latter give off diverticula to communicate with the cavities of numerous processes of the body—the socalled feet-which are the chief locomotive organs of the adult. The radiating and circular vessels, with all their appendages, constitute what is known as the "ambulacral system;" and in Asterids and Echinids this remarkable system of vessels remains in communication with the exterior of the body by canals, connected with perforated portions of the external skeleton-the so-called "madreporic canals" and "tubercles." In Ophiurids the persistence of any such communication of the ambulacral system with the exterior is doubtful, and still more so in Crinoids. In Holothurids no such communication obtains; the madreporic canals and their tubercles depending freely from the circular canal into the perivisceral cavity.'

By Professor Wyville Thomson the larva of the Echinodermata is termed the pseud-embryo;' since it leads a perfectly independent existence, and the true Echinoderm is usually 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, immovably connected, calcareous plates. The intestine is convoluted, and there is a distinct anus. The mouth is usually armed with calcareous teeth, and is always situated on the inferior surface of the body, but the position of the anal aperture varies. The larva is pluteiform and has a skeleton.

The 'test' of the Echinoidea is composed of numerous calcareous plates, firmly united to one another by their edges, and bearing different names according to their position aud function. In all recent members of the order the test is composed of twenty rows of these plates, arranged in ten alternating double rows of plates, which pass from the one pole of the animal to the other. Five of these double rows are composed of large plates, which are not perforated by any apertures (fig. 32); the zones formed by these imperforate plates being termed the '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 is composed of two rows of small plates, which are perforated by minute apertures for the emission of the 'ambulacral tubes,' or 'tube-feet.' Growth of the test is carried on by additions made to the edge of each individual plate, by means of an organised membrane which passes between the sutures, where the plates come into contact with one another. The plates of the test are studded with large tubercles, which are more numerous on the inter-ambulacral areas than on the ambulacral, and are wanting on all the plates which do not belong to either area. These tubercles carry spines, used defensively and in locomotion, which are articulated to their apices by means of a sort of 'universal,' or 'ball-and-socket,' joint. Occasionally a small ligamentous band passes between the head of the tubercle and the centre of the concave articular surface of the spine, thus closely resembling the 'round ligament' of the hip-joint of man. Besides the main rows of plates just described, other calcareous pieces go to make up the test of an Echinus. The mouth is surrounded by a coriaceous buccal membrane, which contains a series of small calcareous pieces, known as the 'oral plates;' whilst a cor

responding series of anal plates' is found in the membrane surrounding the opposite termination of the alimentary canal, In addition to these minute ossicula, at the summit of the test is the apical disc,' composed of the so-called genital and ocular plates (fig. 32, 3). The 'genital plates' are five large plates of a pentagonal form, each of which is perforated by the duct of an ovary or testis. One of the genital plates is larger than the others, and supports a spongy tubercle, perforated by many minute apertures, like the rose of a watering. pot, and termed the 'madreporiform tubercle.' The genital plates occupy the summits of the inter-ambulacral areas. Wedged in between the genital plates, and occupying the summits of the ambulacral areas, are five smaller, heart-shaped, or pentagonal plates, known as the 'ocular plates,' each being perforated by a pore for the reception of an 'ocellus' or 'eye.'

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1. Portion of the test of Galerites hemisphericus enlarged, showing the inter-ambulacral area (a), and the ambulacral areas (b). 2. Galerites hemisphericus viewed from above. a. Inter-ambulacra; b. Ambulacra. 3. Genital and ocular disc of Hemicidaris intermedia, enlarged. c. Ocular plate; d. Genital plate; e. Anal aperture; f. Madreporiform tubercle. 4. Spine of the same. (After Forbes.) The tubercles are mostly omitted on figs. 2 and 3 for the sake of clearness.

Besides the spines, which are sometimes of a very great length, the test often bears curious little appendages, called pedicellaria,' and often supposed to be parasitic. Each of these consists of a stem, bearing two or three blades or claws, which snap together and close upon foreign objects, like the beak of a bird. Their action appears to be independent of the will of the animal, and their true function is not known. Locomotion in the Echinoidea is effected by means of a