Original Articles.

LXXII.-ON THE EMBRYOLOGY OF THE ECHINODERMATA. By Professor Wyville Thomson.*

VI. THE EMBRYOGENY OF THE ECHINIDEA.

41. So far as we are as yet aware, the mode of reproduction in this group is much more uniform than in the ASTERIDEA. In all the species which have been hitherto studied, the nutrition and locomotion of the nascent embryo are provided for by a highly developed pseudembryo, corresponding essentially with the Bipinnarian form, but differing from it in several remarkable details. The earlier developmental stages have only been traced with any degree of certainty in two families of the ECHINIDEA, the Echinide and the Spatangide; and only in detail in three genera, Echinus, Echinocidaris, and Spatangus. Among these there is no essential difference in general plan of development, though certain marked distinctions in detail of form and structure appear to stamp the pseudembryo in each case with definite family or generic characters.

All the Echinidean pseudembryos agree in the possession of one character, which they share with those of the OPHIURIDEA alone. A somewhat complicated framework of delicate, hollow, calcareous rods is developed within the substance of the zooid, supporting the various arms and appendages, and giving a certain amount of rigidity to the whole organism. They likewise agree in the excessive development of the posterior region of the body, which forms an arched dome or a lengthened cone, all the appendages which correspond with the soft lateral appendages of Bipinnaria being thrown forward as a diverging row of rigid tentacles surrounding the mouth, though not surrounding it symmetrically.

42. Von Baer, Dufossée, Derbès, Krohn, and Müller have succeeded in inducing artificial impregnation in this group, so that the development of the pseudembryo has been traced through its earliest phases. Von Baer's observations were made at Trieste, in 1845,† on

Concluded from Nat. Hist. Rev. 1868, p. 415.

† Bulletin de la Classe Physico-Mathematique de l'Academie des Sciences de Saint-Pétersbourg, t. v. p. 234. Froriip's Neue Notizen, XXXIX, p. 36.

Echinus lividus (Lam.) and Echinus brevispinosus (Risso). They are more imperfect than those of Krohn and Müller, as they do not extend beyond the period of hatching. The researches of M. Dufossée were published in 1847;* his results have not been verified. In 1847,† M. Derbès published an elaborate memoir on the embryogeny of Echinus brevispinosus (Risso). M. Dèrbes, from his previous experience in comparative physiology, was thoroughly equal to his task, and his observations were careful and exact. Some monstrous or abortive forms seem to have been produced during the development of some of his broods, and he appears to have fallen into the error of regarding them as normal developmental stages. His observations generally have been thoroughly verified by the subsequent entirely independent results of Krohn and Müller. Müller believes, from the form of the young larvæ, that Derbès' observations refer to Echinus lividus (Lam.), and not to E. brevispinosus (Risso). Derbès' young certainly seem to differ greatly from those procured by the artificial impregnation of the latter species, but from the greater resemblance of the mature forms, I am inclined to think it more probable that they belong to E. pulchellus, whose pseudembryos are at that age scarcely distinguishable from those of E. lividus.

43. Echinus lividus is the species whose development has been most carefully traced; I shall therefore attempt, by combining the researches of several observers, and especially of Krohn and Müller, to describe in detail the development and relations of the pseudembryo in this species. Before doing so, however, I shall describe briefly a characteristic mature form of Echinid pseudembryo, that the relations of the parts may be more clearly understood.

An amount of confusion, which it is extremely difficult to eliminate, has arisen from the odd circumstance that all the figures in Müller's classical Memoirs on these Zooids represent them up-sidedown. In this position they singularly resemble a tent, or a painter's easel, stretched by a varying number of ropes, or resting on several, usually four, feet. Terms such as "tent-arms," "easel-rods," &c. founded upon these resemblances, and of course all involving a complete inversion of the animal, confuse the nomenclature excessively. I shall select, as an illustration of the mature form, a pseudembryo

• Annales des Sciences Naturelles, t. vii. p. 44, 1847.

† Observations sur les Phénomènes qui accompagnent la Formation de l'Embryon chez l'Oursin comestible. Annales des Sc. Nat. t. viii. p. 80 (1847.)

observed by Müller at Heligoland, in 1845, and afterwards at Elsinore in 1847.* This form may almost certainly be referred to Echinus Sphaera (Müll.). I have only met with it on one occasion, and that was in the Frith of Forth, where I took it with the towing net, in a locality where this species only was abundant.

44. The body of the zooid is somewhat quadrangular, and of a glassy transparency. The inferior extremity (Fig. 7, 6) is domeshaped, the superior surface is excavated. The four angles of the body are produced upwards, into four straight, delicate, somewhat divergent appendages (f, h). The quadrangular body has two opposed wider surfaces, the posterior (c) and the anterior (d), and

FIG. 7.-Pseudembryo of Echinus Sphaera (Müll.), observed at Heligoland in 1845-a, mouth; b, inferior extremity; c, posterior surface; d, anterior surface; e, "echinoderm disk;" f, posterior body-appendages; g, anterior body-appendages; h, primary oral appendages; k, accessory oral appendages; 7, ciliated fringe; m, ciliated epaulettes; n, arms; o, granular hood covering the stomach; p, rods supporting the primary oral appendages; q, pedicellariæ; r, rod supporting one of the posterior appendages. (After Müller.)

• Ueber die Larven und die Metamorphose der Ophiuren und Seeigel. Berlin,

1848.

two narrower, lateral surfaces. The appendages (f) limiting the posterior surface, I shall call the posterior, and the opposite pair the anterior appendages. Between the posterior appendages, the hyaline substance of the body is prolonged into a loose crenated border, while between the anterior arms it is produced into a long process, which bears the mouth and oesophagus. Four appendages, the primary oval (h) and the accessory oral appendages (k), ter minate the buccal process; the number of processes surrounding the mouth is thus increased to eight. The whole body of the pseudembryo is traversed by a delicate calcareous framework. Two hollow rods (r), slightly denticulated, run from the posterior extremity of the body to the ends of the two posterior appendages. A second pair, each branching from one of the first pair, about the middle of the body, pass to the ends of the primary oral appendages, a third pair, entirely independent, run from the apex of the dome to the ends of the anterior appendages; while a fourth pair, connected together, and forming an arch, support the accessory oral appendages.

Four transverse, somewhat reniform pads, covered with long cilia to which Müller has given the name of "ciliated epaulettes" (m), are placed on the surface of the body, one at the base of each of the four body appendages. Beneath each epaulette there is a deep yellow patch of pigment. Each appendage is bordered on either side by a ciliated fringe (1). These fringes unite at the end of each appendage, and pass from one appendage to another by fringing the margin of the vault. Between the appendages, however, the fringe does not precisely follow the margin; thus, on the lateral surfaces, and especially on the left side, it sinks on the surface of the vault, till it forms an arch, whose apex nearly reaches the inferior extremity. The ciliated fringe passes round the appendages of the buccal process. The mouth (a) is triangular, bounded above by a transverse projecting lip; beneath, the two sides form a prolonged angle. It is surrounded by a special ciliated fringe. The buccal cavity is continued downwards into a large muscular oesophagus, which communicates with a round stomach occupying the centre of the dome. There is a marked constriction at the oesophageal opening of the stomach, and a second at the point where the stomach passes into a short intestine, which curves slightly upwards, and opens by a small round anal aperture near the centre of the posterior surface. These zooids move freely in the water by means of their vibratile

cilia. They swim uniformly with the crown of appendages in advance. The appendages are rigid, rendered so by the included calcareous rods. The accessory oral appendages are, however, frequently thrown into violent passive motion by the contractions of the oesophagus.

45. In this condition the pseudembryo is apparently a perfect animal, performing with activity all the functions of organic life. It absorbs nourishment apparently through the whole surface, but more especially through the alimentary tract. It moves gracefully, though it may be impossible to say whether its movements are voluntary or automatic. It is highly irritable, retreating from the slightest touch. It appears to be sensible to the stimulus of light. All the species tend to congregate towards the lightest side of a tank. So far as its own life and organization are concerned, it appears to be thoroughly independent of the species from which it sprung, and which it is its function to perpetuate. It frequently lives for weeks without undergoing any change, and then dies. Under ordinary and favourable circumstances, however, it undergoes a change essentially analogous to the change undergone by Bipinnaria.

I shall return, in a subsequent paragraph (§ 62), to the metamorphoses of this species; but, in the first place, I shall describe in detail the formation of the pseudembryo in the species in which its development has been most carefully observed.

46. In the year 1848, Dr. August Krohn,* who had previously succeeded in the artificial impregnation of the eggs of Echinus, undertook the investigation by this means of the Embryogeny of Echinus lividus (Lam.), a common Nice form. During the months of February, March, and April, six trials were made, and three of these proved more or less successful. Dr. Krohn's observations were carried further than those of M. Derbès.

47. The pseudembryo leaves the vitelline sac an extremely immature organism, with the assimilative organs as yet entirely wanting; later, when these are formed, there is a greater necessity for special nourishment, and the want of a due supply in confinement usually causes the death of the greater number of the young, or the development of the pseudembryos is preternaturally slow. Among the great number of young produced at one impregnation, all are not developed to the same degree. This may arise from the eggs not

* Beitrag zur Entwicklungsgeschichte der Seeigellarvern. Heidelberg, 1849.