vni 



PHYLUM MOLLUSCOIDA 



349 



-.- 



FIG. 290. Phoronis aus trails, free end, 

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

 mouth; nephr. nephridial aperture; neph. 

 nephridium. (After Ben ham.) 



tent 



loped from ova. But multiplication by fission has been proved 



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



of the association is effected in this manner throughout the group. 



Each 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, wo., 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 ;y, 

 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 nephri- , 

 dial tubes (neph.) of mesodermal 



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



organisation, af. bl. afferent blood vessel ; , , . , ,-, J , 



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



epistome; mes. mesentery : mo. mouth ; n T, Qrn -U or ^f f^ /Wlnrnp 



nephr. p. nephridiopore ; nephr. d. duct Chamber OI tne CCBlOme. 



of nephridium; nephrost nephrostome The CGelome, which is lined With 



(internal opening of nephridium) ;3s. 0280- i -IT T-IT 



phagus ; red. rectum ; rect. mes. rectal a COelomiC epithelium, IS divided 



(cufshoS: llf^BenhLmf '* 3 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. 



oes 

 efbl 



