In all these cases the egg develops inside the parent, and it was hardly known, before the publication of the interesting researches of M. Prouho,[[572]] that some of the Polyzoa lay eggs which develop externally. In these cases a considerable number of eggs are produced simultaneously by a single zooecium. M. Prouho further throws light on a much contested subject; namely, the nature of the so-called "intertentacular organ" (i, Fig. 234, p. [469]), described so long ago as 1837 by Farre,[[573]] but looked for in vain by the majority of later observers.
The failure to find this organ, even in species which possess it, in certain individuals, according to Farre's statements, is now satisfactorily explained by M. Prouho, who shows that while it is absent in a large number of polypides, it is normally present in those individuals which possess an ovary, and in those only; and that its primary function is that of an oviduct.
The intertentacular organ is an unpaired ciliated tube, which is situated between the two tentacles which are nearest to the ganglion. In the retracted condition of the polypide, it opens from the body-cavity into the tentacle-sheath; and in the expanded condition, directly to the exterior.
In the remarkable case of Alcyonidium duplex, each zooecium normally possesses two sexual polypides. The first of these produces a testis and then becomes a "brown body." The second is meanwhile developed, and produces an ovary and an intertentacular organ, a structure which was not present in the male polypide. The eggs pass through the intertentacular organ into the tentacle-sheath, and attach themselves to the diaphragm (d, Fig. 234), where they remain during their development.
Although the intertentacular organ has been found by Prouho in female polypides only, it would perhaps be going too far to assert that it is confined to polypides of that sex. Hincks[[574]] has observed the passage of spermatozoa in enormous numbers through the organ, although it may be noted that there is no sufficient proof that eggs were not present as well in these zooecia. It further appears that in some cases waste matters may be removed from the body-cavity through the same passage.
It may be presumed that the egg is normally fertilised by a spermatozoon, although this is at present largely a matter of inference. It is believed by Joliet[[575]] that fertilisation is reciprocal, although Prouho has come to the opposite conclusion. Joliet has, however, very justly pointed out that the enormous number of spermatozoa developed by a single individual would be disproportionately large, if their function were merely to fertilise the ovum in the same zooecium. According to his view, the egg is fertilised by a spermatozoon after it has passed into the tentacle-sheath or ovicell, or some other place where it is in free communication with the outside water.
Development and Affinities.—Few parts of the history of the Polyzoa are more fascinating than that which deals with their development; and it is probable that no other is capable of giving so much insight into the affinities of the several groups to one another and to other groups of the animal kingdom.
Fig. 252.—Diagrams of larvae. A, Loxosoma, × 208; a, anus; b, brain, with left eye and ciliated pit; c, ciliated ring; ep, epistome; m, mouth; o, oesophagus; st, stomach; x, aboral adhesive organ: B, Cyphonautes larva of Membranipora (Electra) pilosa, × about 90; a, m, o, st as in A; c, anterior part, and c', posterior part of the ciliated ring; e, epidermis; ms, adductor muscle of shells; p, pyriform organ, of unknown function; sh, shell; v, vestibule; the "internal sac" or sucker, by which fixation is effected, is seen between a and ms. (B, after Prouho.)
The comparative study of the larvae of the Polyzoa may be said to date from 1877, when J. Barrois published an elaborate Monograph[[576]] on this subject. Although some of Barrois' earlier opinions have been subsequently modified, this work still gives the best figures of the external form of the beautiful larvae of many genera. A detailed account of the larval forms of Polyzoa must be omitted from want of space; and the general conclusions only can be given.