vm 



PHYLUM MOLLUSCOIDA 



349 



FIG. 290. Phoronis australis, free end, 

 magnified, an. anus ; ep. epistome ; mo. 

 mouth; nephr. nephridial aperture; neph. 

 nephridium. (After Benham.) 



tent 



loped from ova. But multiplication by fission lias been proved 



to occur actively in one species, and probably increase in tlie numbers 



<l the association is effected in this manner throughout the group. 



Eacli worm is enclosed in a 



membranous or leathery tube, 



within which it is capable of 



being completely retracted. 



The body, which varies in 



length in the different species 



from 1-5 to 127 mm., is cylin- 

 drical, elongated, and unseg- 



mented. 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. 290, 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 distinguished 

 as the oral, the opposite as the 

 anal. A broad lobe the epistome 

 (ep.) overhangs the mouth and 

 lies between it and the anus. Near 

 the anus open two ciliated nepJiri- 

 dial tubes (neph.) of mesodermal 



FIG. 29i. Phoronis australis, internal origin, which open internally each 



organisation, af.bl. afferent blood vessel : , . 



an. anus : ef. bl. efferent blood vessel ; ep. by tWO apertures into the posterior 



epistome; mes. mesentery : mo. mouth ; P l 1fln - l V >pr n f 4-V| p rvWInmp 



nephr. p. nephridiopore ; nephr. d. duct ( 



of nephridium; nephrost nephrostome The CO3lome, which is lined with 



(internal opening of nephndmm); oes. o?so- . '. , ,. . ,. ., 



phagus ; red. rectum ; reel. mes. rectal a CCelonilC epithelium, IS divided 



mesentery ; sept, septum ; tent, tentacles , t i 



(cut short). (After Benham.) into two parts of very unequal ex- 



"tent by a partition or mesentery 



which runs across just behind the tentacles. The anterior part is 

 in communication with cavities in the tentacles and the epistome. 



