Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/453

Rh P L Y Z A 433 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 (Gymuolsema) remarkable bud-like structures termed ovecia 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 &quot;larva&quot; swims freely in its early condition by means of cilia, and is in this condition a single polypide or &quot; person.&quot; The forms assumed by these ciliated larvte 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 zooecium 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 hi/bernacula. 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, Freshwater Polyzoa (Ray Society, 1856); Hincks, British Marine Polyzoa (Van Voorst, 1880); Haddon, &quot;Budding in Polyzoa,&quot; Quart. Journ. Micr. Sci., 1883 ; Balfour, Embryology, vol. i. p. 242 ; and the original memoirs cited by these writers. THE VERMIFORMIA. 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 uf organization, and correlatively with that in the fact that it does not produce buds. Further, some Sabel- 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 Chcetopods la). Like some Sabella3, 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- lus), and anteriorly provided with a horse - shoe - shaped crown of tentacles surrounding the mouth (figs. 4, 5). There is an inter- tentacular &quot; web &quot; between the bases of the tentacles as in the Phylactolse- ma. Caldwell (6) has recently shown that the tentacles are supported by a mesoblastic skele ton, as is also the case in Rhabdo- pleura, but appar 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, i). 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 pra?- oral lobe or &quot; epi- stome &quot; (figs. 4, 5, &amp;lt;:). This organ is aborted in Paludicella, and in deed in all the Gym- nolaima, but is present in the other Polyzoa, i ( * 11 1 FlG. &amp;gt;. i.ain ;u view ui me un cin i ana IS especially large i&amp;gt;) loron j s . The tentacles of the right arm of the and well developed in lophophore are cut short in ordei- to expose clearly .,,., , , i /-&amp;lt; &quot;ie mouth 6 and the overhanging &quot;epistome&quot; iiliabdopieura and L/6- O r prse-oral lobe c . e, intestine; h, dorsal vessel, phalodiscus. 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 prai-oral XIX. -- 55 six times linear (from Allman). , horse-shoe- shaped 1 iphophore with tentacles; c, epistome (prse-oral lobe or prostomium); &amp;lt;/. oesophagus; /, ventral vessel; g, g, two anterior vessels which unite to form /; i, longitudinal muscular coat of the body-wall ; t, intertentacular membrane.