[151] Kowalevsky states that what I have called the mouth is the anus, but his subsequent descriptions shew that he has transposed the mouth and anus in the embryo, and that the opening, which he asserts to be the anus, is in reality the mouth. [FIG. 163. A SERIES OF STAGES IN THE DEVELOPMENT OF PHORONIS FROM ACTINOTROCHA. (After Metschnikoff.) A. Young larva. B. Larva after the formation of post-oral ring of tentacles. C. Larva with commencing invagination to form the body of Phoronis. D. Invagination partially everted. E. Invagination completely everted. _m._ mouth; _an._ anus; _iv._ invagination to form the body of Phoronis.] The youngest free larva observed by Metschnikoff (No. 373) was less developed than the oldest larva found by Kowalevsky. It probably belongs to a different species. The body is uniformly ciliated (fig. 163 A). There is a large contractile præ-oral lobe, and the body ends behind in two processes. The mouth (_m_) is ventral, and the anus (_an_) dorsal, and not terminal as in Kowalevsky's larva. The alimentary tract is divided into stomodæum, stomach and intestine. The two processes at the hind end of the body are the rudiments of the first-formed pair of the arms which are so characteristic of the fully developed Actinotrocha. A second pair of arms next become established on the dorsal side of the previously existing pair, and the region where the anus is placed grows out as a special process. New pairs of arms continue to be formed in succession dorsalwards and forwards, and soon constitute _a complete oblique post-oral ring_ (fig. 163 B). They are covered by long cilia. Round the anal process a very conspicuous ciliated ring also becomes established. At the period when five pairs of arms are present a delicate membrane becomes visible on the ventral side of the intestine which joins the somatic mesoblast anteriorly. This membrane is the rudiment of the future ventral vessel. The somatic mesoblast is present even before this period as a delicate layer of circular muscular fibres. When six pairs of arms have become formed an involution (fig. 163 C, _iv_) appears on the ventral side, immediately behind the ring of arms. This involution consists both of the epiblast and somatic mesoblast. It grows inwards towards the intestine, and, increasing greatly in length, becomes at the same time much folded.