Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/454

434 P L Y Z A 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 epistome of the Phylactoliuma (fig. 11, e) and the buccal shield of Rhabdopleura (fig. 7, d) and of Cephalodis- cus (fig. 9, I) 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, /, y, h). A pair of ciliated canals acting as genital pores is found near the anus ; these have been shown by Caldwell to 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 Actinotrocha (fig. 6). This larva possesses a hood-like region overhanging A A. a B Flo. fi. Development of Phoronis and typical ciliatc larvae. (1), (2), (3), (R), < 9 )> 10), stages in the development of Phoronis (1), earliest larva; ( .), lateral view of the Actinotrocha; (3), ventral view of the same; (8), the ventral in vagination iv is formed; (9), the ventral invagination is everted, carrying with it a loop of intestine; (10), the permanent relations of mouth, anus, imd body (I odaxonia) are attained. (4), (5), KchinoJerm larva with architroch, as in Actinotrocha, but band-like, not digitate. (a), Echinoderm larva, with the architi odi divided into aprse-oral cephalotroch (Molluscan and Rotifer s velum), and a post-oral branchiotroch. (7), Chsetopod ti ochosphere larva with ceplialo- troch only, and elongation and segmentation of the oro-anal axis, a, anus; o, mouth ; ;>r, prostomium ; iv, vential invagination of Phoronis larva. A IJ. oro- anal axis; V D, dorso-ventral axis. the mouth and a number of ciliated post-oral processes or tentacles. The anus is placed at the extremity of the elongate body opposite to that bearing the mouth and prte-oral hood. The prte-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 pne-oral hood to the anus is thus the true dorsal surface of Phoronis, whilst the elongated body is an outgrowth of the ventral surface perpendicular to the primary oro-anal 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 Siplinculus. On the other hand Echiurus, the Chietopods, Nernertine 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 Jackson). THE PTKROBRANCHIA. This section of the Polyzoa also comprises forms which differ very widely from Paludicella. Inasmuch as their development from the egg is at present quite unknown, it may possibly prove that they have other affinities. Only two genera are known, Rhabdopleura (Allman) and Cephalodiscus (M In tosh), the former dredged by Dr Norman in deep water off the Shetlands (and subse quently in Norway), the latter taken by the " Challenger " expedition in 250 fathoms off the coast of Patagonia. The Pterobranchia have the mouth and anus closely approximated, and immediately below the mouth are given off a series of ciliated tentacles, but these do not form a complete circle as in Paludiculla, nor is the lophophore (the platform of their origin) horse-shoe-shaped as in Phoronis. The lophophore is drawn out into a right and a left arm in Rhabdopleura (fig. 7), upon each of which are two rows of ciliated tentacles ; no tentacles are developed centrally in the region between the two arms, so that the mouth is not completely surrounded by these processes. The horse shoe-shaped lophophore of Phoronis could be modified so as to represent the tentaculiferous arms of Rhabdopleura by suppressing both rows of tentacles at the curve of the horse-shoe, and leaving only those which occur on the arms or rami of the horse shoe (see fig. 4). The lopho phore of Cephalodiscus presents us with twelve processes, each carrying two rows of ciliated tentacles ; in fact we have six pairs of tantaculiferous arms instead of a single pair, and each of these arms is precisely similar to one of the arms of Rhabdopleura (fig. 9), excepting that it terminates in a knob instead of tapering. There is no arrangement for introverting the anterior portion of the body into the hinder portion in the Pterobranchia. The little epistome or pra>oral lobe of Phoronis is repre sented in the Pterobranchia by a large muscular shield or disk-like structure (fig. 7, d and fig. 9, />) which over

hangs the mouth and has an actively secreting glandular