genital glands, and therefore only four genital plates in the apical disc.

As regards their development, most of the Echinoids pass through a metamorphosis, as spoken of previously in treating of the development of the class. In these cases the larva is so unlike the adult animal that it was originally described as a distinct animal under the name of Pluteus, from its resemblance to a painter's easel (fig. 82). The larva exhibits bilateral symmetry, and is furnished with provisional organs in the shape of ciliated epaulettes, a skeleton of calcareous rods, and an alimentary canal. The adult Echinoid is developed out of a portion of its substance only; and the rest of the larva is absorbed or thrown off. In some Echinoids, on the other hand, the process of development is direct, and there is no "Pluteus " stage, but the young animal is produced viviparously, and simply requires to grow to be converted into the adult. Sir Wyville Thomson has, further, shown that the young are in many of these cases carried by the mother for a certain period of time in a kind of nursery or marsupial pouch, formed principally by special arrangements of the spines covering some particular portion of the test.

The typical Sea-urchins are divided into the two great groups of the "Irregular" and "Regular" Echinoids (or the Echinoidea exocyclica and Echinoidea endocyclica). The Irregular Echinoids have the anus situated outside the apical disc, marginal or sub-marginal in position, and have only four genital plates. They are also mostly destitute of a masticatory apparatus; and are generally of an oblong, pentagonal, heart-shaped, or discoidal figure (as in the common Heart-urchins " and "Cakeurchins "). The "Regular" Echinoids, on the other hand, have the anus placed at the summit of the test, surrounded by the genital disc; the test is almost always circular or spheroidal; and the mouth is armed with a complicated masticatory apparatus.

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Another singular group is that of the Echinothuridæ, in which the test is "regular," but the plates of both the ambulacral and inter-ambulacral areas are imbricated and overlap one another, rendering the test quite flexible. The existing genera, Calveria (or Asthenosoma) and Phormosoma, and the Cretaceous genus Echinothuria, belong to this group.

A fourth group of the Echinoids is that of the Perischoëchinida, which is not only extinct, but is wholly confined to the Palæozoic period. In all these ancient forms there is the peculiarity that the test consists of more than twenty rows of plates, there being a multiplication of either the inter-ambulacral or

the ambulacral plates, though there are still only five inter-ambulacral and five ambulacral areas. Thus in Archæocidaris, Palachinus, Lepidechinus, and Eocidaris, the ambulacral areas agree with those of the recent Urchins in being composed of only two rows of plates; whilst there are from three to eight or more rows of plates in each inter-ambulacral area. On the other hand, in Melonites and Oligoporus, the ambulacral areas consist, respectively, of ten and four rows of plates. In some of the Perischoëchinide the plates of the test are joined by their edges, as in the common living Urchins; but in others (e.g., Lepidechinus) the plates overlap in an imbricating manner, as in the recent Echinothuride, and the test thus becomes flexible.

CHAPTER XX.

ASTEROIDEA AND OPHIUROIDEA.

ORDER ASTEROIDEA (Stellerida).-This order comprises the ordinary star-fishes, and is defined by the following characters: -The body (fig. 87) is star-shaped or pentagonal, and consists of a central body or "disc," surrounded by five or more lobes or "arms," which radiate from the body, are hollow, and contain prolongations of the viscera. The body is not enclosed in an immovable box, as in the Echinoidea, but the integument (“perisome") is coriaceous, and is strengthened by irregular calcareous plates, or studded by calcareous spines. No dental apparatus is present. The mouth is inferior, and central in position; the anus either absent or dorsal. The ambulacral tube-feet are protruded from grooves on the under surfacee of the rays. The larva is vermiform, and has no pseudembryonic skeleton.

The skeleton of the Asteroidea is composed of a vast number of small calcareous plates, or ossicula, united together by the coriaceous perisome, so as to form a species of chain-armour. Besides these, the integument is abundantly supplied with spines, tubercles, and "pedicellariæ." Lastly, the radiating ambulacral vessels run underneath a species of internal skeleton, occupying the axis of each arm, and composed of a great number of bilateral "vertebral ossicles" or calcareous plates. which are movably articulated to one another, and are provided with special muscles by which they can be brought together or drawn apart. The upper surface of a star-fish corresponds

to the combined inter-ambulacral areas of an Echinus, and exhibits the aperture of the anus (when present), and the

"madreporiform tubercle," which is situated near the angle between two rays. The inferior or ventral surface corresponds to the ambulacral areas of an Echinus, and exhibits the mouth and ambulacral grooves.

The mouth is central in position, and is not provided with teeth; it leads, by a short gullet, into a large stomach, from which a pair of sacculated diverticula are prolonged into each ray. A distinct intestine and anus may, or may not, be present; but the anus is sometimes wanting (in the genera, Astropecten, Ctenodiscus, and Luidia).

The ambulacral system is essentially the same as in the Echinoidea, and is connected with the exterior by means of the "madreporiform tubercle," or "nucleus," two, three, or more of these being occasionally present. The conical or cylindrical ambulacral tube-feet are arranged in two or four rows, along grooves in the under surface of the arms (fig. 88). Each ambu

lacral groove is continued along the lower surface of one of the arms, tapering gradually towards the extremity of the latter. The floor of each

groove is constituted by a double row of minute calcareous pieces-the "ambulacral ossicles" - which are movably articulated to one another at their inner ends. At the bottom of each groove is lodged one of the radiating canals of the watervascular system or ambulacral system, from which are given off the rows of suctorial feet, or "tube-feet."

Fig. 88.-Diagram of a star-fish (Goniaster), showing the under surface, with the mouth and ambulacral grooves. a Ambulacral ossicles, with the ambulacral pores between them: 6 Adambulacral plates, bounding the ambulacral grooves; m Marginal plates (wanting in many species); o Oral plates, placed at the angles of the mouth.

It follows from this that the radiating vessels of the ambulacral system are outside the chain of ambulacral ossicles, so that these latter are to be regarded as an internal skeleton, and they do not correspond with any part of the skeleton of Echinoids--at least they do not correspond with the perforated ambulacral plates of the Sea-urchins. The ambulacral ossicles, however, of the Star-fishes are of such a form that by their apposition an aperture or pore is formed between each pair. By means of these pores (fig. 88, a) the tube-feet communicate with a series of little bladders or "ampullæ," placed above the chain of ossicles. These perforations, however, do not correspond with the perforated plates of the Echinoid test, and the tube-feet of the Star-fishes pass through no "poriferous" plates on their way to the exterior.

This may be rendered more intelligible by examining a section of the arm of a Star-fish from which the soft parts have been removed (fig. 89). In such a section the ambulacral ossicles (a a) are seen in the centre of the lower surface, united along the middle line by their inner extremities. They are so placed as to form a kind of elongated pent-house, and immediately beneath the line where the ossicles of one side are articulated with those of the other side is placed the ambulacral vessel (6). Superficial to this, again, is a nerve-cord; so that the whole chain of ambulacral ossicles is placed in the midst of the soft parts of the animal, and is thus clearly an internal skeleton. At their outer extremities the ambulacral ossicles are articulated by the intervention of the "adambulacral plates" (fig. 88, 6), with plates belonging to the external or integumentary skeleton. As before said, the shape of the ambulacral ossicles is such that a pore is formed by the apposition of each pair; and by these apertures each tube-foot communicates with a vesicle placed internal to the chain of ossicles. It will

* The structures in the Echinus, which are truly homologous with the ambulacral ossicles of the Asteroidea and Ophiuroidea, are the so-called "auriculæ."

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be seen, however, that the tube-feet (indicated by the dotted lines in the figure) do not pass through these apertures, or through any other pores of the skeleton, on their way to the surface. The "poriferous zones" of the Sea-urchins are part of the external skeleton, and are not represented in the Star-fishes. On the other hand, the integumentary skeleton in the Star-fishes is absent along the ambulacral areas, or along the areas occupied by the ambulacral grooves.

Nerve-cord. The

Fig. 89.-Section of the ray of Uraster rubens. a a Ambulacral ossicles; & Position of the ambulacral vessel; cc Plates of the external skeleton; dotted lines show the tube-feet proceeding from the ambulacral vessel.

The blood-vascular system consists, as in the Echinus, of two circular vessels, one round the intestine, and one round the gullet, with a dilated tube, or heart, intervening between them. There are no distinct respiratory organs, but the surfaces of the viscera are abundantly supplied with cilia, and doubtless subserve respiration; the sea-water being freely admitted into the general body-cavity by means of numerous contractile ciliated tubes, which project from the dorsal surface of the body.

The nervous system consists of a gangliated cord, surrounding the mouth and sending filaments to each of the rays. At the extremity of each ray is a pigment spot, corresponding to one of the ocelli of an Echinus, and, like it, supposed to be a rudimentary organ of vision. The eyes are often surrounded by circles of movable spines, called "eyelids."

The generative organs are in the form of ramified tubes, arranged in pairs in each ray, and emitting their products either into the surrounding medium, by means of efferent ducts which open round the mouth, or into the general body-cavity, by dehiscence, the external medium in this latter case being ultimately reached through the respiratory tubes. In their development, the Asteroidea show the same general phenomena as are characteristic of the class; but the larvæ are not provided with any continuous endoskeleton. In some Asteroids the larval forms have side-lappets, and have been described