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of deep-sea animals published by his father, M. Sars, in 1868 as _Halilophus mirabilis_, a name which has been superseded by _Rhabdopleura normani_, Allman,[39] based on specimens dredged by Canon Norman in 90 fathoms, off the Shetland Islands. {22}[Illustration: 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.)] {23}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.