170 



P O L Y Z A 



As negative characters it is important to note the absence 

 of all trace of metameric segmentation, of setse, 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 aivhi- 

 troch (see Lankester, "Notes on Embryology and Classification," 

 Quart. Journ. Micr. Sci., 1876), having a pros-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 actmotrocha (Phoronis) larva is readily comparable with the 

 trochosphere larva? of Echinoderms, Chsetopods, Gephyrreans, 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 excepted) the 

 predominant one. The Fhoronis 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 

 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). t', vestibule 

 (the cup-like depression of the tentaculiferous end of the body) ; in, mouth ; 

 /, digestive gland ; uu.i, anal invagination ; fg, the ciliated disk (corresponding 

 to the cement gland of Loxosoma (fig. 16, s/is) ; x, so-called "dorsal organ," 

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

 ganglion. 



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 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 

 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 of the larva. FIG. 19. Diagram of an 



Two of the most remarkable forms of free- 

 swimming larva; of Gynmolffima 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. 



(from Balfoui). 

 anus ; m, mouth ; st, 

 stomach ; 5, ciliated 

 ; in figs. 17, IS, 



_ 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- 

 Isema 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. -,._, . . 



3 F:G. 20. Larva of Ahyomdium mytih (from Balfour 

 iy represents an an- after Barrois). m 1, problematic "structure; st, oral 

 cestral condition of invngination (?) = Harmer's cephalic ganglion; s, cili- 

 the Polyzoa. Pro- ated disk (corresponding to/.? 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 

 pive-oral ciliated band of 

 the trochosphere larva of 

 Eehiurus, Polygordius, 

 Cluetopods, and Mollus- 

 ca, he pointed out that in 

 both eases the ciliated 

 girdle divides the larva 

 into a hemisphere in 

 which month 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 

 nrsp-nval Inhp Rut in IGl 21 -~ Lal ' va cf MemDrampora (known as Cy 

 >oe. But, ac- plKmautcs) m> mouth ; a', amis j /?, ciliated 

 cording to Balfour s body; a-, problematical body, supposed by Bal' 

 theory, in Polyzoa it is f ur * ^ e a Dut l> similar to the dorsal organ 2 

 the second hemisphere in "e 9 - 17 . 18 - 4 ' either s/ or m in flg. 20. 

 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 Ascidians. In- 

 genious as this speculation is, we 

 must remember that it takes nn 

 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 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. 



Riiationa of the Po!y:oa to the 

 Brachiopoda. 



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 

 to the lophophore, the nephridia, 

 ami the termination of the intes- 

 tine as have the gastro-parietal 

 and ilio-parietal bands or mes- 

 enteries of the Terebratulidse. 

 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 



fa 



Jf, seta.- at the margin of the calyx ; p, 

 stalk comparable to the stalk ot Pedi- 

 cellina, Loxosoma, Cephalodiscus, and 

 Rhabdopleura;pa, cement gland ut the 

 apex of the stalk (after Moise). 



