P O L Y Z O A 



163 



Paludicella, as we have seen, develops both ova and 

 spermatozoa in one and the same polypide. The details 

 of impregnation and development have not been followed 

 in this instance, but in some of the marine Eupolyzoa 

 (Gymnolaema) remarkable bud-like structures termed oaecia 

 are developed for the special reception of the ova, and in 

 these organs fertilization takes place. In the Entoprocta 

 there is a peculiar brood-pouch. The spermatozoa of one 

 polypide probably in all cases fertilize the ova of another, 

 but we have not yet in many cases a knowledge of how 

 the spermatozoa get to the eggs, or how the eggs escape 

 from the body-cavity of the parent. In the hippocrepian 

 freshwater Polyzoa (Phylactolsema) the ova appear to be 

 fertilized and undergo the early stages of development 

 within the body-cavity of the parent or in a hernia-like 

 protrusion of it. Probably in such cases the embryos 

 escape by the death of the parent and rupture of the 

 parental tissues, as do also the peculiar asexual internal 

 buds or statoblasts of these forms. 



The embryo Polyzoon or "larva" swims freely in its 

 early condition by means of cilia, and is in this condition 

 a single polypide or "person." The forms assumed by 

 these ciliated larvae in different Polyzoa are very various 

 and exceedingly difficult of interpretation. We shall have 

 more to say with regard to them below (see figs. 19, 20, 

 21). The ciliated larva then fixes itself and commences 

 to produce polypides by a process of budding, the buds 

 remaining not merely in contact but in organic continuity, 

 and increasing continually in number so as to form a large 

 colony or zoarium. In Paludicella we have seen that this 

 colony has a simple tree-like form. The new buds form 

 as wart-like growths, usually one, sometimes two in number, 

 at the free end of a cell or zocecium near the spout-like 

 process from which the tentacular crown is everted. In 

 Paludicella all the polypides of a colony are alike ; there 

 is no differentiation of form or distribution of function 

 amongst the members of the colony. In many Eupolyzoa 

 this simplicity is by no means maintained, but a great 

 variety of form and function is assumed by various 

 members of the aggregate. The only approach to a 

 differentiation of the polypides in Paludicella is in the 

 arrest of growth of some of the buds of a colony in 

 autumn, which, instead of advancing to maturity, become 

 conical and invested with a dark-coloured cuticle. They 

 are termed hybernacida. Should the rest of the poly- 

 pides die down in winter, these arrested buds survive 

 and go on to complete development on the return of 

 spring. 



In Paludicella we have thus seen a fairly simple and 

 central example of Polyzoon structure and life-history. 

 The variations upon this theme presented in different 

 groups of Polyzoa have been to some small extent noted 

 in the preceding account, but we shall now be able to 

 indicate them more precisely by considering the various 

 groups of Polyzoa in succession. The limit assigned to 

 this article necessitates very large omissions. The reader 

 who wishes to have the fullest information on the many 

 difficult and uncertain matters connected with this subject 

 is referred to Allman, Freshieater Polyzoa (Ray Society, 

 1856) ; Hincks, British Marine Polysoa (Van Voorst, 

 1880); Haddon, "Budding in Polyzoa," Quart. Journ. 

 J/tcr. Set., 1883 ; Balfour, Embryology, voL L p. 242 ; and 

 the original memoirs cited by these writers. 



THE VERMIFORMLL 



The first section of the Polyzoa comprises but a single 

 genus, Phoronis. It differs from all other Polyzoa first 

 in its greater size (species 2 inches long are known) 

 and elaboration of organization, and correlatively with 

 that in the fact that it does not produce buds. Further, 



it does not produce a closely adherent cuticular zooecium 

 as do Paludicella and the Eupolyzoa generally, but a 

 leathery tube in 

 which the animal 

 freely moves, resem- 

 bling that of some 

 Chastopods (Sabel- 

 la). Like some 

 Sabellse, Phoronis 

 forms closely packed 

 aggregates of indi- 

 viduals not brought 

 together by any 

 process of budding, 

 but each separately 

 developed from an 

 egg. Phoronis has 

 an elongate, worm- 

 like, unsegmented 

 body, with a conical 

 posterior termina- 

 tion (like Sipuncu- 

 1ns), and anteriorly 

 provided with a 

 horse - shoe - shaped 

 crown of tentacles 

 surrounding the 

 mouth (figs. 4, 5). 

 There is an inter- 

 tentacular " web " 

 between the bases 

 of the tentacles as 

 in the Phylactolae- 

 ma. Caldwell (6) 

 has recently shown 

 that the tentacles 

 are supported by a 



mesoblastic skele- F '- *->* hippotrepia, Wright; magnified 



. sii times linear (from Allman). a, horse-shoe- 



ton, as IS also the shaped lophophore with tentacles; c, epistome 



Aflp in TtViahHn- (P-oral lobe or prostomium); rf, (esophagus; /, 



XVnaOQO- Tentra i Terae l: g, g, two anterior vessels which 



pleura, but appar- nnite to form /; i, longitudinal muscular coat 



. , of the body-wall ; t, intertentacular membrane. 



ently not the case 



in any other Polyzoa. Close to the mouth, as in all 

 Polyzoa, is placed the anus, outside the horse-shoe-shaped 

 lophophore or tenta- 

 cular platform (fig. 

 11, t). The tenta- 

 cular crown is not 

 introversible ; in this - 

 point Phoronis differs 

 from Paludicella and 

 the Ectoproctous Eu- 

 polyzoa, and agrees a _ 

 with the Entoprocta 

 and the Pterobranchia. 

 Overhanging the 

 mouth is a small pree- 

 oral lobe or " epi- 

 stome " (figs. 4, 5, c). 

 This organ is aborted 

 in Paludicella, and in- 

 deed in all the Gym- 

 nokema, but is present 

 in the other Polyzoa, 



j ii ' Fio. 5. Lateral view of the anterior region of 



and IS especially large Phoronis . The tentacles of the right arS of the 

 and well developed in lophophore are cut short in order to expose clearly 

 T>I ! j i j /-i the mouth ft and the overhanging "epistome" 



Knabaopleura and Oe- or pite-oral lobe c. c, intestine ; A, dorsal Teasel. 

 phalodlSCUS. It has Other letters as in fig. 4. 



been compared to the Molluscan foot, but undoubtedly in 

 Phoronis it is the persistent representative of the prae-oral 



i . 



1 



