The embryo early becomes a "blastosphere" or hollow vesicle formed of a single layer of cells. One half of this is invaginated, or pushed into the other half, and a "gastrula" is thus developed, the cavity of which is the "archenteron," and the two cell-layers respectively "ectoderm" and "endoderm." The "blastopore," or orifice of invagination, is at the posterior pole of the larva, where it narrows and closes, the locomotor, transverse band of cilia developing round it. No other bands of cilia appear in this form of development. The proboscis becomes marked out externally by the appearance of a circular groove, near the middle; and behind this groove a second one appears, which forms the posterior boundary of the collar. The larva, which now resembles Fig. 8 C, is usually hatched at this stage. Two gill-slits make their appearance, and the mouth and anus are perforated; the anus being in the position of the blastopore.
The body-cavities are formed as five derivatives of the archenteron. One of these is unpaired, and becomes the proboscis-cavity; while the others are the paired cavities of the collar and trunk (cf. Fig. 2). There is some uncertainty about the origin of the body-cavities of the free-swimming Tornaria, although it seems most probable that they are developed either from the wall of the stomach or intestine,[[35]] or from scattered mesoderm cells[[36]] which lie in the segmentation-cavity.
The metamorphosis of Tornaria is accompanied by a great diminution in size,[[37]] probably due to the loss of water; by this cause and by the simultaneous thickening of the skin, the larva loses its transparency.
The external features of the metamorphosis are sufficiently indicated by Fig. 8, the ciliated bands finally disappearing. The dorsal pore persists as the proboscis-pore; the notochord and numerous gill-slits are developed as outgrowths of the alimentary canal, the reproductive organs make their appearance, probably from the mesoderm,[[38]] the trunk meanwhile elongating so that the proportions of the adult are acquired.
Order II. Pterobranchia.
Tubicolous Hemichordata, with one pair of gill-slits or none, a U-shaped alimentary canal, and a dorsal anus situated near the mouth. Proboscis flattened ventrally into a large "buccal disc," its base covered dorsally by the collar, which is produced into two or more tentaculiferous arms. Trunk short, prolonged into a stalk. Reproduction by budding occurs.
This group consists of the two genera Cephalodiscus (Fig. 9) and Rhabdopleura (Fig 12). The latter, first dredged by G.O. Sars, in 1866, from 120 fathoms off the Lofoten Islands, was included in a catalogue of deep-sea animals published by his father, M. Sars, in 1868 as Halilophus mirabilis, a name which has been superseded by Rhabdopleura normani, Allman,[[39]] based on specimens dredged by Canon Norman in 90 fathoms, off the Shetland Islands.
Fig. 9.—Cephalodiscus dodecalophus, M‘Intosh, Straits of Magellan; A, small portion of the common "house," × 1; a, a single individual, shown also as B, × 65; six of the tentacular arms, belonging to the collar, are seen springing from behind the proboscis or "buccal disc." This has a crescentic band of pigment parallel with its posterior border, which conceals the mouth. The stalk, bearing a bud, which already shows the beginning of two tentacular arms, is seen to the right. (After M‘Intosh, B from Parker and Haswell.)
The structure of Rhabdopleura has been described by Sars,[[40]] Lankester,[[41]] and Fowler.[[42]] R. normani is common in certain Norwegian Fjords, at depths of 40 fathoms or more, and has been recorded by Fowler from the Tristan d'Acunha group in the S. Atlantic; R. compacta has been found off the N.E. coast of Ireland[[43]] and near Roscoff, on the N. coast of Brittany; while forms described by Jullien[[44]] as R. grimaldii and R. manubialis have been dredged off the Azores. I have recently found a fragment of Rhabdopleura from South Australia. It is doubtful how far these species are distinct.