350 



ZOOLOGY 



SECT. 



to the Phylactolsemata, it is most commonly looked upon as 

 related to that class and to the Brachiopoda, and the Phoronida 

 may thus conveniently be dealt with as a class of the Molluscoida. 

 Phoronis (Fig. 284) lives in associations consisting of a number 

 of individuals, all of which are developed from ova, there being no 

 process of asexual formation of buds. Each worm is enclosed in a 

 membranous or leathery tube, within which it 

 is capable of being completely retracted. The 

 body is cylindrical, elongated, and unsegmented. 

 At one end there is a crown of numerous 

 slender, ciliated tentacles borne on a horse-shoe- 

 shaped lophophore, the lateral cornua of which 

 are spirally coiled in the larger species ; these 

 are supported by a mesodermal skeleton and 

 are non-retractile. 



Both mouth and anus (Fig. 285, mo, an) are 

 situated at this tentacular extremity of the 

 body, separated from one another by only a 

 short space. This short space between mouth 

 and anus represents, as in the Polyzoa, the 

 greatly abbreviated dorsal surface; but it will 

 be convenient to term this end of the animal 

 the anterior, and the opposite the posterior 

 end : the side of the elongated body towards 

 which the mouth is approximated may be dis- 

 tinguished as the oral, the opposite as the 

 anal. A small lobe — the epistome {ep) — over- 

 hangs the mouth and lies between it and the 

 anus. Near the anus open two ciliated ne- 

 phridial tubes (neph) of mesodermal origin, 

 which open internally each by two apertures 

 into the posterior chamber of the ccelome. 



The ccelome, which is lined with a ccelomic 

 epithelium, consists of three main parts of 

 very unequal extent. The first {prosoccele) is 

 a narrow cavity in the epistome. The second 

 (mesoccele), which is in communication with the 

 first, lies in front of a transverse septum or 

 mesentery extending between the mouth and 

 anus, and perforated by the oesophagus but 

 not by the rectum ; it is prolonged round the lophophore and 

 gives off narrow diverticula to the hollow tentacles. The third, 

 and by far the most extensive part of the ccelome {inetacozlc), 

 occupies the whole of the length of the body behind the trans- 

 verse septum. It is subdivided into two by a median longi- 

 tudinal mesentery (Fig. 287, m, m.), which extends from the oral 

 to the anal surface and supports both limbs of the alimentary 



Fig. 284.— Phoronis 

 australis, natural 

 size. 



