has also increased in depth, reaching two or three cells thickness, while the central lumen has assumed a three-lobed shape. The embryo is now becoming more transparent, so that it is possible to utilise whole mounts if well cleared. A slightly older stage, when the larva has reached its fullest development as a pluteus, is drawn

Fig. 12.—Fully-developed larva, with embryonic attachment.
Post.P., posterior pole; Rt., Rt., Skel.Pl., left, and right skeletal plate of larva; Stom., stomach; Hydr., hydrocoel; Ant.P., anterior pole; Oes., oesophageal sac; Mesench., mesenchyme; Skel.Rud.Ad., skeletal rudiment of the adult; Emb.Al., embryonic attachment.

in Fig. 12, from a cleared whole object. The pluteus has a somewhat pyriform shape, rather pointed at the anterior (oral) pole, and rounded in the posterior part to which is joined the embryonic attachment. The oesophagus in the specimen figured is somewhat elongate in form, but this character is variable, some specimens having the organ rounded. The wall of the oesophagus is thickened and its lumen consequently very narrow. To the left is seen the five-lobed hydrocoel with the rudiments of the ambulacral canals visible within the five lobes. The large, spherical stomach occupies most of the aboral part of the larval body. The ectoderm is in the form of a thin sheet of investing cells of regular arrangement. On either side of the stomach extend forwards the two vestigial skeletal rods of the pluteus, whilst triradiate rudiments of some of the plates of the future adult are to be seen scattered in the more superficial regions of the mesenchyme just below the ectoderm.

The relationship between this peculiar reduced and vestigial larval stage and a typical pelagic Ophiopluteus is illustrated in diagrammatic form in Fig. 13, where it can readily be seen that the pluteus found in Amphipholis squamata is in reality an armless version of the strongly bilaterally symmetrical larva of those types of ophiuroids which have an indirect, pelagic development. As is shown elsewhere (Fell, 1945), this vestigial larva of Amphipholis can be regarded as but one of a series of forms which illustrate various stages in the gradual reduction and disappearance of the larva in the ontogeny of ophiuroids. It has been one of the purposes of this study

Fig. 13.—Diagram illustrating the relationship of the vestigial pluteus of Amphipholis squamata (right) to the fully-developed ophiopluteus of ophiuroids with indirect development (left).

to obtain information on such a reduced larva as this in order to achieve some further degree of understanding of the manner in which the larva has been so completely lost by Kirk's ophiuroid. While the general relationships of the reduced pluteus of Amphipholis squamata are more fully discussed elsewhere, it will be useful to note here the more salient points in which it differs from the pelagic Ophiopluteus. These are:—

• (1) All four pairs of larval arms have disappeared, leaving only traces of their skeletal rods in the two calcareous plates on either side of the stomach.

• (2) The pluteus here is not free-living, being formed within the bursa of the parent, and attached to it by an outgrowth of cells from the wall of the bursa. This organ is termed here the “embryonic attachment.”

• (3) The tissues are more heavily charged with yolk material. Nutrition is obtained partly from this substance, and partly from secretions of the bursal wall (see below), and thus a pelagic food-gathering stage is rendered physiologically unnecessary.

• (4) As a condition correlated to the previous one, the alimentary system is not yet functional. The oesophageal sac is still closed off from the exterior, and there is no anus developed.