Fig. 131. Free-swimming larva of Alcyonidium mytili. (After Barrois.)
m (?) dorsal organ; st. stomodæum (?); s. ciliated disc.
The larva, having now acquired all the important structures it is destined to possess, becomes free. It is shewn in [fig. 131]; the oral face being turned upwards. There are two rings of cilia, one round the edge of the ciliated disc, and a second with larger cilia on the ring of large cells described above. This ring projects somewhat; its projecting edge being directed towards the ciliated disc. The dorsal organ (m?) is placed on the oral face at the bottom of an elongated groove, in front of which is a bunch of long cilia or flagella. Two long flagella are also developed at the posterior extremity of the oral face, and two pairs (an anterior and a posterior) of eye-spots also appear. Towards the posterior extremity of the oral face is seen a body marked st, which forms the internal sack. If I am right in regarding this as the stomodæum, it is probable that it never unites with the invaginated hypoblast, and that the alimentary tract of the larva remains therefore permanently in an imperfect condition.
Careful observations have been made by Repiachoff (No. [318]) on the early development of Tendra, which accord in some respects with the results arrived at by Barrois in his second memoir. The observations are not, unfortunately, carried down to the complete development of the larva.
The ovum divides in the normal way into two and then four uniform segments. These four next become divided by an equatorial furrow into four dorsal and four ventral segments, the former constituting the aboral pole and forming the epiblast, and the latter the oral pole. The stages with sixteen and thirty-two cells appear to be formed in the same manner as in Alcyonidium—but between the two layers of cells forming the oral and aboral poles a well-marked segmentation cavity arises at the stage with sixteen segments. At the stage with thirty-two cells the four middle cells of the oral side, which are larger than the others, become divided into two tiers, in such a manner as to form a prominence projecting into the segmentation cavity. By the appearance of a lumen in this prominence it becomes converted into an archenteron, which communicates with the exterior by a blastopore in the middle of the oral surface. The blastopore becomes eventually closed.
The archenteric sack of Repiachoff is clearly the same structure as Barrois’ four invaginated cells of the oral face, their further history has unfortunately not been followed out by Repiachoff.
The free larva swims about for some time, and then fixes itself and undergoes a metamorphosis; but the exact course of this metamorphosis is still very imperfectly known.
According to the latest statements of Barrois the attachment takes place by the oral face[124]. The ciliated disc, which in the free larva forms a kind of cup directed towards the aboral end, turns in upon itself towards the oral face. It subsequently undergoes degeneration and forms a nutritive or yolk-mass. The skin of the larva after these changes gives rise to the ectocyst or cell of the future polype. The future polype itself appears to originate, in part at any rate, from the so-called dorsal organ[125].
The first distinct rudiment of the polype appears as a white body, which gradually develops into the alimentary canal and lophophore. While this is developing the ectocyst grows rapidly larger, and the yolk in its interior separates from the walls and occupies a position in the body cavity of the future polype, usually behind the developing alimentary canal. According to Nitsche (No. [316]) it is attached to a protoplasmic cord (funiculus) which connects the fundus of the stomach with the wall of the cell. It is probably (Nitsche, etc.) simply employed as nutritive material, but, according to Barrois, becomes converted into the muscles, especially the retractor muscles.
Adopting the hypothesis already suggested in the case of the Entoprocta the metamorphosis just described would seem to be a case of budding accompanied by the destruction of the original larva.
This view of the nature of the post-embryonic metamorphosis is apparently that of Claparède and Salensky, and is supported by Claparède’s statement that the formation of the first polype ‘resembles to a hair’ that of the subsequent buds. The mode of budding would, however, appear to present certain peculiarities, in that the whole larval skin passes directly into the bud, while from the rudimentary bud of the larva the lophophore and alimentary tract only of the fixed polype are formed.