164 



P O L Y Z O 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 Phylactokema (fig. 11, e) and the 

 buccal shield of Rhabdopleura (fig. 7, d) and of Cephalodis- 

 cus (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, /, </, 

 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. 



^ -. 



(3) 



(10) 



- D 



FIG. 6. Development of Phoronis and typical dilate lame. (1), (2), (3), (8), 

 (9), (10), stages In the development of Phoronis (1), earliest larva; (2), lateral 

 view of the Actinotrocha; (3), ventral view of the same; (B), 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, and body 

 (Podaxonia) are attained. (4), (5), Echinoderm larva with architroch, as in 

 Actinotrocha, but band-like, not digitate. (6), Echinoderm larva, with the 

 architroch divided into aprte-oral ccphalotroch (Molluscan and Rotifer's velum), 

 and a post-oral branchiotroch. (7), Chsetopod trochosphere larva with cephalo- 

 troch only, and elongation and segmentation of the oro-anal axis, a, anus; o, 

 mouth; pr, prostomium ; iv, ventral invagination of Phoronis larva. A B, oro- 

 anal axis; VD, dorso-ventnil 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 



prse-oral hood. The prse-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 prse-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 Sipunculus. 

 On the other hand Echiurus, the Chaetopods, Nemertine 

 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 PTEROBRANCHIA. 



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'Intosh), 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 Paludicella, 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 arc 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 prse-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, b) which over- 

 hangs the mouth and has an actively secreting glandular 



