phoronis body axis mouth
POLYZOA 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 of organization, and correlatively with that in the fact that it does not produce buds. Further, it does not produce a
closely adherent cuticular zoweium as do Paludicella and the Eupolyzoa generally, but a leathery tube in which the animal freely moves, resem-bling that of sonie Chictopeds la). Like some Sabelke, 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 tertnina-tion (like Sipuncu-lus), and anteriorly provided with a horse - shoe - shaped crown of tenta,cles surrounding the mouth (figs. 4, 5). There is an inter-tentacular " web " between the bases of the tentacles as in the Phylactolm-ma. Caldwell (6) has recently shown that the tentacles are supported by a mesoblastic skele-ton, as is also the case in
Ithabdo-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 Plioronis differs from Paludicella and the Ectoproetous Eu-polyzoa, and agrees with the Entoprocta and the Pterobranchia. Overhanging the mouth is a small prie-oral lobe or "
epi-stome " (figs. 4, 5, c). This organ is aborted in Paludicella, and in-deed in all the Oym-tiolmina, but is present in the other Polyzoa, and is especially large ' and well developed in Pthabdopleura and Ce-phalodiscus. It has lobe of the larva (fig. 6), and therefore cannot be compared ' to the _Molluscan foot. If we are right in associating Phoronis with the Polyzoa, this fact is sufficient
to show that the epistoine of the Phylactoheina (fig. 11, e) and the buccal shield of Rhabdopleura (fig. 7, (/)and of Cephalodis-curs (fig. 9, b) are also cephalic in nature, and cannot rightly be identified with the post-oral and ventral muscular lobe known as the foot in Mollusca. A circum-oral nerve ring occurs at the base of the tentacles and sends off a cord which runs along the left side of
the body-. The alimen-tary canal presents the same general form and regions as in Paludicella. It hangs in the body-cavity, to the walls of which it is suspended by definite mesenteries.
Phoronis presents a closed contractile vascular system containing red-coloured blood-corpuscles (figs. 4, 5, f, g, h). A pair of ciliated canals acting as genital pores is found near the anus ; these have been shown by Caldwell t3 be typical nephridia.
The development of Phoronis is remarkable. The egg gives rise (after the usual phases of cleavage and gastrula-tion) to the larval form known as Actinotrocka, (fig. 6). This larva possesses a hood-like region overhanging prz,e-oral hood. The prze-oral hood becomes the epistome, and the tentacles, by further development (new
tentacles replacing the larval ones), become the horse-shoe-shaped group of tentacles of the adult. A very curious process of growth changes the long axis of the body and results in the anus assuming its permanent position near the mouth. An invagination appears on the ventral face of the larva between the anus and mouth, and attains con-siderable size. At a definite moment in the course of
growth this invagination is suddenly everted, carrying with it in its cavity the intestine in the form of a loop. Thus a new long axis is suddenly established at right angles to the, original oro-anal axis, and continues to de-velop as the main portion of the body. The short area extending from the prm-oral hood to the anus is thus the true dorsql surface of Phoronis, whilst the elongated body is
an outgrowth of the ventral surface perpendicular to the primary oro-aual axis, as conversely in many Mollusca we find a short ventral area (the foot) between mouth and anus, and an outgrowth of the dorsal surface (the visceral hump) perpendicular to the primary oro-anal axis, forming-the chief body of the animal. In these relations Phoronis (and with it the other Polyzoa) agrees with Sipanculus.
On the other hand Echiurns, the Clactopods, Netnertine worms, and some other groups which start from a simple larval form not unlike that of Phoronis, present a continual elongation of the original oro-anal axis, and no transference of the long axis by the perpendicular or angular growth of either the ventral or the dorsal surface of the larva.
Phoronis was discovered originally in the Firth of Forth by- Dr Strethill Wright. It occurs in the Mediterranean and in Australian seas (Port Ja-ckson).
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