slender chord surrounding the mouth (g), from which three delicate filaments are sent off opposite the base of each ray: the middle one is continued along the middle of the ambulacral groove, and swells, according to Ehrenberg, into a small terminal ganglion, immediately behind that bright coloured speck at the extremity of the ray which the same acute observer regards as a rudimental organ of vision.

The organs of generation consist of groups of ramified tubes (fig. 64. d), arranged in pairs in each ray, and opening upon the calcareous circle which surrounds the mouth. In the males these sacculi are distended with a white fluid abounding in spermatozoa: in the females they are laden with ova of a bright yellow or orange colour, which distend the rays during the breeding season.

The five pairs of generative organs are restricted to the central disc in the Ophiura, which part in the breeding season is distended with the milky fluid of the testis in the male, and with the round yellow eggs in the female. They are discharged by orifices on the ventral surface. In the Comatula the ovarian receptacles are much more numerous, and are of smaller size: they occupy the inner side of each of the pinnæ or articulate processes sent off from the rays.

Echinida. The calcareous pieces entering into the composition of the complex skeleton of the Echinus are those of the shell, of the buccal apparatus called the "lantern," of the ambulacral tubes, and of the pedicellariæ.

All the Echini are admirable for the regular and beautiful pattern in which, as in a tesselated pavement, the numerous calcareous pieces composing their globular crust are arranged; many of the species are formidable from the size and form of the spines with which the shell is beset. The component plates of the shell are divided into several series, called oral, anal, genital, ocular, ambulacral, and interambulacral plates. The proper shell, one half of which is exposed by removal of the spines in figure 65, is built up of the two latter kinds, which constitute a hollow spheroid, having a large aperture at each pole,

of the spines, are characterised by more numerous tubercles, and are not perforated. Both kinds of plates are of a pentagonal form, and are arranged each kind in five alternate pairs of vertical rows. The plates of each pair are united together by a zigzag suture, and increase in size as they approach the equator of their living globe. These twenty series of ambulacral and interambulacral plates constitute the chief part of the spheroidal skeleton of the Echinus. The large oral aperture is partly occupied by the small irregular oral plates, which have no tubercles or spines, and are suspended in the oral integument, from the middle of which project the points of the five teeth. At the opposite aperture, immediately surrounding the vent, are the small anal plates; external to these are the five genital or oviducal plates, so called because each is perforated by the duct of an ovarium or testis; the ocular plates are wedged into the external interspaces of the genital plates, and are pierced near the apex by a very minute pore, which lodges the ocellus and its little nerve.

One of the genital plates is larger than the rest, and bears a tubercle corresponding with the nucleus or madreporiform tubercle on the back of the star-fish. M. Agassiz, assuming this plate to be at the back part of the Echinus, shewed that the other four genital plates were in symmetrical pairs, and thus discovered the right and left sides of the animal.

The calcareous constituent of the shell of the Echinus lividus, has the following chemical composition, according to the analysis of Professor Brunner, quoted by Professor Valentin.*

The small anal plates are united together like the oral ones by an extensile and contractile membrane. Both the internal and external surface of the rest of the complicated shell is covered by a similar organised membrane, which likewise extends through all the numerous sutures of the shell. With this explanation of the general structure of the crust of the Echinus we are in a condition to understand the manner of its growth, which otherwise would be a difficult physiological problem.

The Echinus maintains nearly the same spheroidal figure from its earliest formation to full maturity; and, notwithstanding that its soft

* Monographies D'Echinodermes, d'Agassiz. No. I. 1841.

parts are almost entirely confined by a fragile and inflexible globular crust, this is never shed and reproduced, like the shells of the crab and lobster. At the same time the calcareous plates possess not more power of inherent growth than the crusts of the Crustacea, which they resemble in both physical and chemical properties. By the subdivision of the hollow globe into many pieces, and the apposition of a formative membrane to all their margins, addition is gradually made to the circumference of each component plate, and by the plan of their arrangement the spheroidal shell gradually expands, with little change in its figure and relative proportions.

The amount of change in the form of the shell, which differs in different species, depends upon the addition of new plates to the ambulacral and interambulacral series. These are developed near the oral and anal poles, but chiefly near the latter, where, in the young Cidaris, for example, the plates are more loosely connected together, and support incomplete spines. In the membrane connecting such plates may be seen small irregular pieces, without tubercles or spines, which grow by accretion to their margins, and then have the tubercles developed upon their outer surface. The spines are at first immoveable, and stand out like processes from the tubercle; the joint is not developed until after they have acquired a certain size. The growth of the globe in the direction of its poles is chiefly by the development of the new plates; its expansion at the equator is by the addition to the sutural margins of the old plates.

The spines of the Echini vary in form and relative size in different genera; their proximal extremity is adapted, by an excavation, to the tubercles on the outer surface of the plate, to which it is attached by a capsular ligament, and upon which it can be rotated by muscular fibres external to the capsule. In the species of Cidaris, where the spines are unusually large, an internal ligament extends from a little pit upon the centre of the tubercle to the centre of the articular cavity of the spine, analogous to the round ligament in the hip joint. The spines grow by successive additions, through calcification of that part of the common organised membranous covering of the shell of the Echinus, which is attached to their base. The varied cellular organisation of the spines, affords beautiful microscopical objects, when viewed in thin transverse slices.

The tubes that issue from the ambulacral pores can be extended beyond the longest spines in the Echinus Sphæra of our own coasts; they terminate in suckers, which appear to be highly sensitive, and by which the Sea-urchin attaches itself to foreign bodies, and moves along them with a rotatory course, in which the spines serve to balance and direct the progress of the animal. The bases of the

tubes communicate with the cavities of the internal vesicles or branchiæ. The terminal sucker of the tube is supported by a circle of five or, sometimes, four reticulate calcareous plates, which intercept a central foramen, and by a single delicate reticulated perforate plate on the proximal side of the preceding group. The centre of the suctorial disc is perforated by an aperture conducting to the interior of the ambulacral tube.

I have reserved the notice of another class of appendages to the integument, not only of the Echini, but of the Asteriæ, for this part of my discourse, because they are most developed, most varied in structure, and have been most minutely investigated in the species of the globular family of Echinoderms. The appendages to which I allude are called "Pedicellariæ," and consist of a dilated end or head, usually prehensile, supported by a slender stem or pedicel. They present different forms, which hold constant and determinate positions in the crust of the Echinus: they seem at no season to be absent, and must therefore form part of the integral organisation of the Echinoderm. They have however been conjectured by some naturalists to be parasitic animals; by others to be the young of the Echini, to which they are attached.

In the Ech. lividus, Professor Valentin, to whom we owe the most minute descriptions of these bodies, divides them into gemmiform, tridactyle, and snake-headed pedicellariæ. They are all composed of an internal calcareous axis, and a soft external tissue.

The gemmiform pedicellariæ * are placed around the tubercles, especially the largest ones; their pedicel is long and slender; their capital resembles the bud of a flower, defended by three sepals, the apex of each of which is produced inwards in the form of two pairs of long and slender teeth. The quadridentate sepaloid plates can be divaricated and approximated, and constitute a very effective prehensile instrument: they are highly irritable; a needle introduced into their grasp is instantly seized. The ciliated gemmule of any parasitic coralline, which might settle about the base of a spine, and there commence its growth, would be liable to be seized and uprooted by the prehensile gemmiform pedicellariæ, which are of microscopic minuteness.

The tridactyle pedicellariæ are of larger size, are visible to the naked eye, and fit to grapple with and dislodge young sedentary parasites of larger species, as Cirripeds and Conchifers. They are found more particularly around the large tubercles of the interambulacral plates which support the largest spines. Their capital is longer, narrower,

*Pedicellaria globifera Muller.

and more pointed than in the gemmiform kind; and the three pieces are dentated and close upon each other, like the blades of pincers. The "pedicellariæ ophicephala" are aggregated principally upon

the buccal membrane.

The pedicellariæ of the star-fishes are diffused generally over the surface, and form dense groups round the spines: they consist of a slender contractile stem; but the head resembles a forceps with two blades they are continually in motion, opening and shutting their blades. They would wage as effective and serviceable a war in defence of the integument of the Asterias against the attacks of the host of parasites which the sea engenders, as their tridactyle analogues in the Echini may do. In some species of Goniaster the pedicellariæ resemble the vane of an arrow, and are so numerous as to give a villous appearance to the integuments.

The muscular system of the Echinus, into the details of which the limits of the present lecture forbid me to enter, includes the muscles of the spines, those of the jaws or lantern, of the buccal membrane, of the anus, of the ambulacral tubes, of the internal branchiæ, and of the pedicellariæ. The muscles of the lantern and spines have their ultimate filaments collected into primitive fibres or fascicles, which are marked by transverse striæ at regular distances as in the muscles of insects. *

The digestive apparatus of the Echinus (fig. 66.) consists of a

Echinus.

mouth armed with teeth, surrounded by a muscular labial membrane, and five pairs of pinnate tubular tentacula, of an oesophagus and stomach, and of an intestine suspended by a mesentery to the interior of the shell, and which, after performing a few circumgyrations, terminates by a distinct outlet opposite to the mouth. The outer margin of the lip is fringed by a circle of the ophicephalous pedicellariæ, visible to the naked eye.

The teeth (a) are five in number; they are calcareous, three sided prisms, dense at the working apex, softer at the base, with the inner

edge sharp and fit for cutting; they are each implanted in a larger