FIG. 15.-Pseudembryo of Echinocidaris equituberculata—a, mouth; b, inferior pole; c, posterior surface; d, anterior surface; e, posterior appendages; f, anterior appendages; 9, primary oral appendages; h, accessory oral appendages; k, diverging "auricular" appendages; 7, anterior pair of prolongations of the margin of the body; n, posterior pair; p, accessory anterior body-appendages. (After Müller).

which they are essentially composed, separate into three diverging branches at their origin in the body of the zooid. These branches do not unite with the rest of the skeleton. The accessory oral appendages are developed as in other Echinoid pseudembryos. Their calcareous rods are as usual arched, and united in front of the oesophagus. From the centre of the arch a simple style dips down into the body. The zooid now possesses ten appendages, and in this condition, it is mentioned by Gegenbauer, as an Urchin-larva, with eight usual and two additional arms. In its mature condition, however, it has yet another pair of short arms, arising between the anterior appendages and the oral process. Their calcareous styles are

derived from the arch of the accessory oral appendages. These lastformed appendages acquire no ciliated fringe. The free edge of the superior surface develops four symmetrical soft ear-like appendages, which bend downwards towards the inferior angle; round these the ciliated fringe passes. The "Echinoderm disk" is developed in the usual position on the left side of the stomach, and pedicellariae are found on the opposite side of the dome as in Echinus. In the farthest developed pseudembryos observed by Müller, the calcareous crown of the side rods of the body had disappeared, but at the same time an areolated calcareous net-work had spread out on either side of the rod somewhat further up in the body, and a like development of perforated plates had taken place from the insertions of the rods of the anterior body-appendages.

69. Minute embryo urchins, whose origin from these pseudembryos was evident from the remains of the calcareous skeleton frequently attached to them, were often taken by Müller in the towing net. They were round and depressed. Upon the dorsal surface were five well-developed sucking feet, with annular calcareous disks to the suckers. On the oral surface slender needle-like spines sprung from a delicate calcareous network, but at the edge there were a series of thick, flattened spines, with a coarse calcareous skeleton. There were also stalked pedicellariae on the oral surface. The flattened spines seemed to correspond with the spatulate spines of Echinocidaris equituberculata.

THE EMBRYOGENY OF THE SPATANGIDÆ.

70. On several occasions, at Heligoland in 1847, and subsequently at Marseilles, at Trieste, and at Nice,* Professor Müller met with Echinoderm pseudembryos resembling generally those of the Echinidea, but distinguished from the ordinary forms by certain characters which they possessed in common. The calcareous rods supporting the anterior and posterior body-appendages were fenestrated; an azygous process supported by a fenestrated rod projected directly downwards from the inferior angle of the body, and they possessed no trace of ciliated epaulettes. In April 1853, M. Krohn† succeeded

Ueber die Entwickelung und Metamorphose der Echinodermen. 1. Abhandl. s. 17, taf. iii., und 4 Abhandl. s. 27, taf. viii. Fig. 10-13.

Müller's Archiv., 1853.

in the artificial impregnation of Spatangus purpureus (O. F. Müller), and his results rendered it highly probable that all the nearly-allied groups of Echinidean pseudembryos without epaulettes, with trellised rods, and with an unpaired apical process might be referred to the Spatangida. For the sake of simplicity I figure one of the earlier developmental stages of Spatangus purpureus. It already shows the well-marked group of characters which seems to be peculiar to the family. After the publication of Krohn's paper, Müller had an ample opportunity of revising his observations on the young of Spatangus at Messina. I abstract the following account of the development of the pseudembryo from the memoirs of Krohn and Müller.

71. Krohn's first figure represents the zooid before the development of the azygous process. At this period it has only the posterior appendages and the rudiments of the primary oral appendages. In his second figure the inferior process, with its calcareous style, is developed, the oral appendages are more distinct. In fig. 4 of Krohn's memoir the anterior body appendages and the accessory oral appendages are formed. Müller's figures of Nice and Trieste forms are nearly identical with those of Krohn. At this period the animal possesses nine arms. The distribution of the skeleton in its earlier stages within the body is the same as in all Echinidean pseudembryos. The mature form observed at Heligoland had thirteen arms, and Müller* concludes that this is the normal number for Spatangid larvæ, and that all forms in which there are fewer than thirteen arms are to be considered immature.

72. The youngest form observed by Müller at Messinat, corresponds with Krohn's first figure, and has as yet developed no apical appendage, the conical body ended simply in a gelatinous point. The two lateral rods branches at first much as in the pseudembryo of Echinus lividus. Four branches are given off on either side. One passes to the end of the posterior appendage, this rod is richly fenestrated. A second style strikes across the body, crossing the corresponding style from the other side above the anus. A third arches forwards towards the primary oral appendage; and a fourth runs down to the inferior angle of the body. A fifth pair of styles shortly

Ueber die Larven und der Metamorphose die Ophiuren und Seeigel. Berlin, 1848.

† Ueber die Gattungen der Seeigellarven. Berlin, 1855.

FIG. 16.-Pseudembryo Spatangus purpureus: a. posterior body appendages; b. oral process; c. rudiments of primary oral appendages; f. azygous inferior appendage. (After Krohn.)

branch from the oral arch, and pass one on either side of the æsophagus towards the apex of the body, thus two pairs of styles, one pair anterior and the other posterior, run towards the inferior angle of the dome. The two styles on each side are bound together by a transverse rod, so that on each side of the body there is a four-sided calcareous frame. When the azygous process is formed, its skeleton consists of a prismatic fenestrated rod, whose base divides into three branches, one running out to either side within the apex of the body; the third is directed forwards. The two side branches gradually extend upwards upon the sides of the dome, till at length they give off calcareous supports to a pair of diverging appendages analogous to the accessory body appendages of Echinocidaris (§ 68). The

anterior style of the apical process remains unaltered. As this arch becomes developed the original lateral calcareous frames are totally absorbed. At the same time another pair of long arms, the anterior body-appendages, arise on either side of the oral process, supported by latticed rods. The development of the arch, supporting the accessory oral appendages, takes place as usual, and somewhat later a pair of accessory anterior body-appendages arise, supported by simple branches from this arch. As in Echinocidaris the ciliated fringe extends over the inferior pair of diverging appendages, it does not occur on the azygous apical appendage, or on the accessory anterior body appendages. In mature individuals, the styles arching to the primary oral appendages develop on either side irregular cribriform calcareous plates. Müller observed some of the more mature forms of this species at Messina, with the rudiments of the Echinoderm disk. The embryo is developed in the usual position; it appears when the pseudembryo has acquired its full complement of thirteen appendages.

73. Müller* refers, with doubt, a beautiful pseudembryo with a globular body, without ciliated epaulettes, with eight appendages, the styles of the four body-appendages fenestrated, and with an elegant reticulation of calcareous rods on the surface of the dome, to Echinocyamus pusillus (Flem.). A good account of the embryogeny of one of the Clypeastride would be an important addition to our knowledge of the subject.

74. In three species of the genus Echinus,-E. lividus, E. pulchellus, and E. brevispinosus, the form of the pseudembryo has been determined by artificial impregnation, and in all three the young larvae, procured by this means, have been connected, by transition stages, with mature sporadic forms. One form, described by Müller, although its parentage has not been actually traced, may almost certainly be referred to E. sphæra. The mature pseudembryos all agree in general form, and in the possession of "ciliated epaulettes" at the bases of the four body appendages. Several other forms have been observed so closely allied as to leave little doubt that they are referable to the same generic type. In certain species, as in E. lividus, E. pulchellus, and in one of the forms observed by Müller, at Heligoland, the body is in the form of an inverted pyramid. Other species, such as E. brevispinosus and E. sphæra

Op. cit.