170 



POLYZOA 



As negative characters it is important to note the absence 

 of all trace of metameric segmentation, of setae, 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 

 Polyzoa, we are not at present able to make use of the facts 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, "Notes on Embryology and Classification," 

 Quart. Journ. Micr. Sci., 1876), having a prse-oral ciliated area 

 (velum or cephalotroch) continuous with a post-oral ciliated band 

 (the branehiotroch), which latter becomes developed into the ten- 

 tacular crown of the adult. 



The actinotrocha (Phoronis) larva is readily comparable with the 

 trochosphere larva; of Echinoderms, Chsetopods, Gephyrseans, and 

 Molluscs. Its special character consists in the strong develop- 

 ment of the post-oral ciliated band, whereas the prse-oral ciliated 

 band is in most other classes (the Sipunculoids exccpted) 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 Podaxouia). 



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 

 diagrammatically 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 Hatschek). r, vestibule 



supposed by Balfour to be a bud, by Haimer (18) regarded as the cephalic 



FIG. 18. Later stage of the same larva as fig. 17. Letters as before, with the 

 addition of nph, duct of the right nephridium ; a, anus ; hg, hind-gut 



the only instance among the Eupolyzoa in which the growth of 

 the diflerent 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 

 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 has not st. 

 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 nf flip lirvn FlG - 19 - Diagram of an 

 wall ot the larva. jdea] Polyzoon larva 



Two of the most remarkable forms of free- ( from Balfour). an, 

 swimming larvse of Gymnohiema 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. 



anus ; m, mouth ; st, 

 stomach ; *, ciliated 

 disk (fg in fig!. 17, 18, 



f'J 



of Membrapora (,,, as Cy . 

 phonautcs). m, month ; a', anus ; fg, ciliated 

 body; x, problematical body, supposed by Bal- 

 four to be a bud, similnr to the dorsal organ 

 in figs. 17, 18, and to either st or m in fig. 'K>. 



Thus tho 



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 Gymno- 

 [sema are extremely 

 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 

 schematized in fig. FlG 50. Larva of Ahyonidium mytili (from Balfour 

 19 represents an an- after Barrois). m 1, problematic structure ; si, oral 

 cestral condition of imagination (?) = Harmer's cephalic ganglion ; s, clli- 

 the Polyzoa Pro- ated disk (corresponding to /y in figs. 17, 18, and 21). 



fessor 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 ~^___^ 



praj-oral ciliated band of ^t* r~-~. 



the trochosphere larva of 

 Echiurus, Polygordius, 

 Chffitopods, and Mollus- 

 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 prostomium or 21 __. 

 prse-oral lobe. But, ac- 

 cording to Balfour's 

 theory, in Polyzoa it is 

 the second hemisphere 



which enlarges and becomes the stalk-like body of the adult, whilst 

 the first hemisphere remains small and insignificant 

 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 Ascidians. 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 larvte of extremely 

 degenerate and peculiar members 

 of the group are not adaptive and 

 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. 



Relations of the Polyzoa to the 



Srachiopoda. 



The Polyzoa were first asso- 

 ciated with the Brachiopoda by 

 H. 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 ^ ^ _ you Terebratulina at a 5ta 

 to the lophophore, the nephndia, wllcn only 5ix tentacles are p,. es ent. 

 and the termination of the lutes- se, setae at the margin of the calyx ; p, 

 tine as have the gastro-parietal stalk comparable to the stalk of Pedi- 

 ; c +ol IIOY,<IC - TUPS cellina, Loxosoma, Cephalodiscus, and 

 and iho-panetal bands or mes- Rhal)lklpleu ,. a . pa ; cen ' len t gland at the 

 enterics of the Terebratulida!. apex of the stalk (after Morse). 

 The young Terebratulina (fig. 22) 



may be 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 



