under the name of Bipinnariæ; and in these, as in the Pluteus of the Echinoids, a large portion of the larva is cast off as useless. In Bipinnaria asterigera (Sars) the digestive cavity is a simple sac which sends no prolongations into the rays, and the mouth is inter-radial, instead of being placed in the centre of the ambulacral system. The mouth of the adult is at this stage closed by the soft external skin of the larva. In other Asteroids the larvæ have three anterior vermiform processes, and are known as Brachiolaria.

The general shape of the body varies a good deal in different members of the order. In the common star-fish (Uraster rubens) the disc is small, and is furnished with long, finger-like rays, usually five in number (fig. 87). In the Cribella the general shape of the body is very much the same. In the Solasters the disc is large and well marked, and the rays are from twelve to fifteen in number, and are narrow and short (about half the length of the diameter of the body). In the Goniasters (fig. 88) the body is in the form of a pentagonal disc, flattened on both sides; the true "disc" and rays being only visible on the under surface of the body. In the singular genus Brisinga, we have in some respects a transitional form between the Asteroids and Ophiuroids, the arms being much longer and more slender than is the case in the typical Asteroids, at the same time that they are much thicker and softer than is the case amongst the latter. In none of the true starfishes, however, are the arms ever sharply separated from the disc, as in the Ophiuroidea, but they are always an immediate continuation of it.

The principal groups of Asteroidea are the following :—

Family . Asteriada or Asterocanthiida.—Four rows of ambulacral feet.

Fam. 2. Astropectinida.-Two rows of ambulacral feet; back flattish, netted with tubercles, which carry radiating spines at the tip ("paxilla").

Fam. 3. Oreastride.-Two rows of ambulacral feet; skin granular, pierced by minute pores.

Fam. 4. Asterinida.--Two rows of ambulacral feet; body discoidal or pyramidal, sharp-edged; skeleton of imbricate plates; dorsal wart single, rarely double.

Fam. 5. Brisingida.-Arms long and rounded, sharply marked off from the disc. Ambulacral grooves not reaching the mouth; two rows of ambulacral feet.

ORDER OPHIUROIDEA.-Body stellate, consisting of a central disc," in which the viscera are contained, and of elongated "arms," which are sharply separated from the disc, solid, not containing prolongations of the viscera, and not furnished inferiorly with ambulacral grooves. Larva generally pluteiform, with a skeleton.

This order comprises the small but familiar group of the "Brittle-stars" and "Sand-stars," often considered as belonging to the Asteroidea, to which they are nearly allied. The body in the Ophiuroidea (fig. 90) is discoidal, and is covered with granules, spines, or scales, but pedicellariæ are wanting. From the body-which contains all the viscera-proceed long slender arms, which may be simple or branched, but which do not contain any prolongations from the stomach, nor have their under surface excavated into ambulacral grooves. The arms, in fact, are not simple prolongations of the body, as in the Asteroidea, but are special appendages, superadded for locomotive and prehensile purposes. Each arm is enclosed by four rows of calcareous plates, one on the dorsal surface, one on the ventral surface, and two lateral. The lateral plates generally carry more or less well-developed spines. In the centre of each arm is a chain of quadrate ossicles, forming a central axis, and between this axis and the row of ventral plates is placed the ambulacral vessel. Each ossicle of the central chain is composed of two symmetrical halves, but these are immovably articulated together, and are not movable upon one another, as in the Asteroidea. The mouth is situated in the centre of the inferior surface of the body, is provided with a masticatory apparatus, and is surrounded by tentacles. opens directly into a sac-like ciliated stomach, which is not continued into an intestine, the mouth serving as an anal aperture. The stomach is destitute of lateral diverticula. The reproductive organs are situated near the bases of the arms, and open by orifices on the ventral surface of the body or in the interbrachial areas. *

The ambulacral system is constructed upon the same plan as in the Echinoids and Asteroids; but its place as a locomotive apparatus is taken by the arms. The radial vessels of the ambulacral system are not provided with secondary vesicles or "ampullæ," as they are in the Echinoidea and Asteroidea, and the lateral "feet" which they give off have no terminal suckers. The madreporiform tubercle is placed on the inferior surface of the body, and is often partially concealed by one of the plates surrounding the mouth.

Respiration is carried on by the lining of the body-cavity, and by a circlet of modified tube-feet or tentacles placed round the mouth. The sea-water is admitted to the body-cavity by means of the genital fissures.

Spontaneous fission has been observed by Lütken and Kowalewsky to take place in some Ophiuroids, as also occasionally in some of the Asteroids.

The development of the Ophiuroids is sometimes direct, the young being brought forth alive, and, in some cases, being carried by the mother for some period after hatching (Wyville Thomson). More commonly there is a pluteiform embryo, which resembles that of the Echinoids in having a continuous endoskeleton.

Fig. 90.-Ophiuroidea. a Ophiura texturata, the common Sand-star;
bOphicoma neglecta, the grey Brittle-star. (After Forbes.)

In Euryale the body is in the form of a sub-globose disc with five obtuse angles, and the arms are prehensile. In Asterophyton, the Medusa-head star, the arms are divided from the base, first dichotomously, and then into many branches. In Ophiura, the sand-star, the arms serve for reptation (creeping), and are undivided, often exceeding the diameter of the disc many times in length.

The order Ophiuroidea may be divided into two families, as follows:

CHAPTER XXI.

CRINOIDEA, CYSTOIDEA, AND BLASTOIDEA.

ORDER CRINOIDEA. -The members of this order are Echinodermata, in which the body is fixed, during the whole or a portion of the existence of the animal, to the sea-bottom by means of a longer or shorter, jointed, and flexible stalk. The body is distinct, composed of articulated calcareous plates, bursiform, or cup-shaped, and provided with solid arms, which are typically from five to ten in number, are independent of the visceral cavity, and are grooved on their upper surfaces for the ambulacra. (The position of the body being reversed, the upper surface is ventral; whilst the dorsal surface is inferior, and gives origin to the pedicle.) The tubular processes, however, which are given off from the radiating ambulacral canals of the Crinoidea, unlike those of the Echinoidea and Asteroidea, are not used in locomotion, but have probably a respiratory function. The mouth is central, and looks upwards, an anal aperture being sometimes present, sometimes absent. The ovaries are situated beneath the skin in the grooves on the ventral surfaces of the arms or pinnules, as are also the ambulacral or respiratory tubes. The arms are furnished with numerous lateral branches or "pinnulæ." The embryo is "free and ciliated, and develops within itself a second larval form, which becomes fixed by a peduncle" (Huxley).

If we take such a living Crinoid as Rhizocrinus (fig. 91), we shall be able to arrive at a comprehension of the leading characters of this order. Rhizocrinus is one of those Crinoids which is permanently rooted to some foreign object by the base of a stalk which is composed of a number of calcareous pieces or articulations. In some cases (as in Apiocrinus) the base of the stem or "column" is considerably expanded. In other cases the column is simply "rooted by a whorl of terminal cirri in soft mud" (Wyville Thomson). The joints of the column are movably articulated to one another, the jointsurfaces often having a very elaborate structure, so that the entire stem possesses in the living state a greater or lesser amount of flexibility. Each joint is perforated centrally by a canal, which is very inappropriately termed the "alimentary canal," but which in truth has nothing to do with the digestive system of the animal. At the summit of the stem is placed the body, which is termed the "calyx," and which is usually

more or less cup-shaped, pyriform, bursiform, or discoidal. The calyx exhibits two surfaces, a dorsal and a ventral, of

which the dorsal is composed, wholly or in part, of calcareous plates articulated by their margins, whilst the former is composed of a more or less leathery integument, strengthened by the deposition in it of numerous small plates of carbonate of lime. The ventral surface exhibits the aperture of the mouth, which may be subcentral or may be very excentric, and which in many extinct forms is wholly concealed from view. The ventral surface also exhibits the aperture of the anus, which is usually placed excentrically in one of the spaces between the arms, and which is often carried at the end of a longer or shorter tubular eminence or process, which is called the "proboscis." Owing to the animal being supported on a stalk, it is evident that the "ventral" surface is turned upwards, and the "dorsal" surface downwards. The column springs from the centre of the dorsal surface; and a stalked Crinoid may

therefore be compared to Fig. 91.-Crinoidea. Rhizocrinus Lofotensis,

a Star-fish turned upside

down, with its lower or ambulacral surface superior,

a living Crinoid (after Wyville Thomson), four times the natural size. a Stem; b Calyx; cc Arms.

and its dorsal surface looking downwards. The calyx contains the digestive canal, and the central portions of the nervous and water-vascular (ambulacral) system; but it does not contain the reproductive organs, as is the case with the visceral cavity of the other Echinoderms.

From the margins of the calyx, where the dorsal and ventral