(524). Little is known concerning the nervous system of the Echini: a few delicate filaments have been observed in the neighbourhood of the oesophagus, apparently of a nervous character, communicating with a nervous ring placed in that vicinity, resembling that already described in Asterias.

(525). The Echini, like the Star-fishes, are bisexual, and in the structure of their reproductive organs display, if possible, greater simplicity than even the Asteridae above described. The ovaria are five delicate membranous bags, quite distinct from each other, that open externally by as many delicate tubes, or oviducts, as we may term them. The apertures through which the eggs escape are easily seen upon the outer surface of the shell, placed around the anus, and are recognizable not merely by their size, but from the circumstance of each perforation being placed in the middle of a distinct oval plate of the shell, distinguished by zoological writers as the ovarian pieces. The membranous sacs in which the ova are secreted vary in size in proportion to the maturity of the eggs contained within them, and at certain times of the year are enormously distended: it is in this state that the "roe of the Sea-egg," as the ovaria are commonly called, is used as an article of food; and in some countries, especially upon the shores of the Mediterranean, they are eagerly sought after, when in season, by divers employed to procure them.

The corresponding organs in the male sex are only distinguishable by the spermatozoa contained in their interior instead of ova.

(526). At the earliest period observed by Muller, the larval Echinus (fig. 99, 1) had the appearance of a transparent dome-like disk, hollowed out inferiorly, and having its margin prolonged into long, slender, diverging processes supported on calcareous pieces deposited in their substance, and giving the whole animal somewhat the appearance of a timepiece standing on many legs (a, b, e, e), four of which (f, e) constitute a sort of framework surrounding the oral apparatus.

(527). The arrangement of the locomotive apparatus of these larvae is very peculiar, consisting of four epaulet-like wreaths of long cilia situated upon the dome-shaped body of the animal, and of numerous ciliated fringes spread over the arms and in the vicinity of the oral organs.

(528). The mouth is a triangular orifice (fig. 99,1, a) furnished with broad lips, and leads immediately into the stomach (d), which is a cul-de-sac, situated in the interior of the body.

(529). In this condition the larvae are not more than half a line in length, and move freely about in the water, rowed along by the action of their cilia, while the marginal processes and other appendages to the body remain quite passive and motionless. The first appearance of metamorphosis is indicated by the development of a shield-like plate (fig. 99, 2, b), which, during the months of August and September, becomes visible beneath the skin covering the dome of the body, sloping as if inclined towards its apex, and not inaptly representing the fingerplate of the timepiece to which, as to its shape, the creature has been already compared. The round shield-like plate thus formed is divided by a cinquefoil-shaped figure into five compartments, and constitutes the first rudiment of the future Echinus; as its size increases, new divisions make their appearance upon its periphery, indicating the situations of the future tentacles or feet; and soon afterwards little round tubercles begin to develope themselves, which gradually rise up into cylindrical elevations and ultimately assume the appearance and texture of the locomotive spines.

Metamorphosis of Echinus. 1. A Pluteus with thirteen arms.

Fig. 99. Metamorphosis of Echinus. 1. A Pluteus with thirteen arms: A A, anterior inferior lateral processes; bb, posterior inferior processes; c c, lateral processes of the vaulted disk; D, terminal process from the apex of the vaulted disk; E E, anterior, and F F, posterior processes of the framework of the mouth; G G, posterior processes of the body: a, mouth; a', basin-like under-lip; I, oesophagus; d, stomach; e, calcareous framework of the skeleton. 2. The same in a more advanced stage of development - the spines of the young Echinus beginning to make their appearance, covered with a transparent skin: a, remnant of the calcareous skeleton of the larva or Fluteus, which has now nearly disappeared; b, branched calcareous spicula belonging to the larva skeleton; c, spines, and d, tentacles of the young Echinus. 3. The echiniform condition almost completed, only a few calcareous spicula of the larva remaining. (After Muller).

(530). The shield itself, forming the basis upon which the apparatus of suckers and spines is supported, is now seen to enclose in its substance its own proper calcareous skeleton: this consists at first of minute detached triradiate spicula, which, as they increase in number, arrange themselves so as to constitute a sort of network in the texture of the skin, wherein ultimately the polygonal calcareous plates of the shell make their appearance.

(531). Holothuridae

The name applied by naturalists to the animals composing the next family of Echinodermata is derived from a Greek word of uncertain application 531 Holothuridae 121 . In common language they are generally known by the appellation of "Sea-cucumbers"; and in fact, to a casual observer, the resemblance which they bear to those productions of the vegetable kingdom, both in shape and general appear-ance, is sufficiently striking. The surface of these animals is kept moist by a mucus that continually exudes through innumerable pores and appears to be secreted by minute follicles imbedded in the substance of the skin. The integument which covers, or, rather, forms the body, is entirely destitute of those calcareous pieces that encase the Echini and Star-fishes; it appears to consist of a dense fibrous cutis of considerable thickness, covered externally with a thin epidermic layer. Beneath the cutis is another tunic, composed of strata of tendinous fibres crossing each other in the midst of a tissue of a semicartilaginous nature, which is capable of very great distention and contraction, and serves by its elasticity to retain the shape of the body. Within this dense covering are seen muscular bands running in different directions, which by their contraction give rise to the various movements of the creature: of these muscles five strong fasciculi assume a longitudinal course, passing along the entire length of the animal from the mouth to the cloaca; and in the interspaces between these, circular and oblique muscles are readily distinguishable.