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.
When it has reached its full development the critical period of the metamorphosis of Actinotrocha into Phoronis is reached, and is completed in about a quarter of an hour. The ventral involution becomes evoluted ([fig. 163] D), just as one might turn out the finger of a glove which had been pulled inwards. When the involution has been to a certain extent everted, the alimentary canal passes into it, and at the same time the body of the larva becomes violently contracted. By the time the evagination is completed it forms ([fig. 162] E) a long conical body, containing the greater part of the alimentary tract, and constituting the body of the young Phoronis. The original anal process remains on the dorsal side as a small papilla ([fig. 162] E, an).