Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/460

440 P O L Y Z O A As negative characters it is important to note the absence of all trace of metameric segmentation, of seta*, and of paired lateral (parapodia of Appendiculata) or median ventral (podium of Mollusca) outgrowths of the body-wall. Larval Forms of Polyzoa. In the consideration of the probable pedigree and affinities of the Polvxoa, we are not at present able to make use of the faets of development from the egg, on account of the extreme difficulty which the study of the young stages of these organisms presents. In the case of Phoronis we have the only readily intelligible his tory. The larva, to start with, is of that form known as an archi- troch (see Lankester, "Xotes on Embryology and Classification," Quart. Journ. Micr. Sci., 1876), having a prre-oral ciliated area (velum or cephalotroch) continuous with a post-oral ciliated band (the branchiotroch), which latter becomes developed into the ten tacular crown of the adult. The actinotroeha (Phoronis) larva is readily comparable with the trochosphere larva? of Echinoderms, Cha>topods, Gephyrreans, and .Molluscs. Its special character consists in the strong develop ment of the post-oral ciliated band, whereas the pra?-oral ciliated band is in most other classes (the Sipunculoids excepted) the predominant one. The Phoronis larva exhibits first of all an oro- anal long axis, and this is suddenly abandoned for a new long axis by the growth of the ventral surface of the larva at right angles to the primary axis (hence the term Podaxonia). In the other Polyzoa we do not at present know of any larva which retains even in its earliest phases the original oro-anal long axis. They all appear to start at once with the peculiar and secondary long axis of the adult Phoronis, so that Balfour has diagrammaticaily represented the Polyzoon larva by the sketch given in fig. 19. This diagram applies, however, more especially to the Entoprocta, since the anus is represented as included in the area of the post-oral ciliated ring. The development of Pedicellina has been very carefully followed by Hatschek, and may be said to be </ Fig. 17. Fig. 18. FIG. 17. Larva of Pedicellina (from Balfour. after Hatschck). v, vestibule (the cup-like depression of the tentaculiferous end of the body) ; m, mouth ; /, digestive gland ; an.i, anal invagination ; /(/, the ciliated disk (corresponding to the cement gland of Loxosoma (tig. 16, ht) , x, so-called "dorsal organ," supposed by Balfour to be a bud, by Haimer (18) regarded as ti>e cejihalic ganglion FIG. 18. Later stage of the same larva as fig. 17. Letters as before, with the addition of />//, duct of the right nephridium ; a, anus ; hrj, hind-gut. the only instance among the Eupolyzoa in which the growth of the different organs and the consequent relation of the form of the larva to the form of the adult is understood (see figs. 17 and 18). In the other Polyzoa, in spite of the painstaking and minute studies of Barrois (14), the fact is that we do not know what face of the larva corresponds to an m the tentacular area, what to the stalk or anti- tentacular extremity, what to the anterior and what to the posterior surface The conversion of the larva into the first polypide lias not st-l been observed in the case of these free-swim ming forms, and it is even probable that no such conversion ever takes place, but that the first polypide forms as a bud upon the body- wall of the larva. Two of the most remarkable forms of free- swimming lame of Gymnolrcma are repre sented in figs. 20 and 21." In both, in addition to the chief post-oral ciliated band, a smaller ciliated ring is observed, which is identified by Balfour with that which is found at the anti-tentacular extremity (base of the stalk) in the Pedicellina larva. Fir.. 19. T)iaeram of an ideal I olyzoon larva (Irotn Balfour). an, anus ; m, mouth ; st, stomach ; *, ciliated disk (fij in figs. 17, 18, and i). It does not seem justifiable, in the face of the existing uncertain ties as to identification of parts, and in view of the high probability that the (iymno- liwma are extremely <s " modified and degen erate forms (a con- sideration which applies in some re spects even more strongly to the En toprocta), to assume that the larval form g schematized in fig. F G 2 o.-Lnn-a of Ar fV oi,li,, n , mvtili (from Balfoir 19 represents an an- after Harrois). m ?. problematic structure.; rt, oral CCStral condition of imagination (?) = Harmer s cephalic ganglion ; .. cili- the Polvzoa Pro- ated disk (corresponding to /p In flgs. 17, 18, and 21). fesi-or Balfour (15) was, however, led to entertain such a view ; and, assuming that the chief ciliated band (drawn as a broad black line) corresponds to the single prie-oral ciliated band of the trochosphere larva of Erbiums, Polygordius, Cha>topods, ana Mollns- ca, he pointed out that in both cases the ciliated girdle divides the larva into a hemisphere in which mouth and anus lie and a hemisphere which is the complement of this ; in most classes the first hemisphere elongates and forms the bulk of the. body, whilst the second hemisphere forms the prostoniium or _ i 1*1 v. t. FIG. 21. Larva of Memm-anipora (known as Cv- prfe.-oral lobe. But, ac- , )honall t es ). ,, mouth; a , anus; */, ciliated cording to BalfoUTS body; x, problematical body, supposed by Hal theory, in Polyzoa it is f(iur t() be a bud, similar to the dorsal organ r the second hemisphere in fi * s - 17 18 and to eitlu r st or * in " -"; which enlarges and becomes the stalk-like body of the adult, whilst the first hemisphere remains small and insignificant. Thus the Polyzoa would fix themselves in later growth by what corresponds to the head or prostomium of other animals, as do the Bar nacles and the Ascidinns. In genious as this speculation is, we must remember that it takes no account of the facts known as to Phoronis, nor of the Ptero- branchia, and that it is con fessedly based upon the assump tion that the larva? of extremely degenerate and peculiar members of the group are not adaptive arid modified, but retain primary and archaic characters. Further, it is to be distinctly borne in mind that the interpretation of parts upon which this speculation rests is, except in the case of Pedicellina, altogether hypo thetical. of tJic Fo7y:oa to fJtc Brachiopoda. The Polyzoa were first asso ciated with the Brachiopoda by 11. Milne-Edwards. The inves tigation of the development of Terebratulina by Morse (16) led to a further perception of the points of agreement in struc ture between these two groups. Lastly, Caldwell (6) has shown that the mesenteries of Phoronis have precisely similar relations to the lophophore, the nephridia, F*-. M--Tg lerc-braful.na . a and the termination of the intes tine as have the gastro-parietal and ilio-parictal bands or mes enteries of the Terebratulidie. The young Terebratulina (fig. 22) may he readily compared with Loxosoma (fig. 16), the peduncle with its cement glands in the former being identical with the, stalk and basal gland of the- latter. The form of the alimentary canal when only six tentacles are present. st, setie at the margin of the calyx : />, stalk comparable to the stalk ot 1 edi- cellina, I.oxo.soma, Ccpha.odisous, and Rhabdopleurajpa, cement gland at the

apex of the stalk (after Morsr).