MOLLUSCA 



553 



of gasteropods called the Opisthobranchiata which show that the 

 changes occurring in torsion are to a certain extent reversible. They 



Fig. 380. Mantle cavities of streptoneurous gasteropods. A, Patella, 

 ctenidia absent. B, Fissurella, ctenidia equal ; kidneys unequal, like those of 

 Patella. C, Haliotis, right ctenidium smaller; ciliary currents shown by 

 arrows, exhalant shown emerging from the three most recently formed holes 

 in the shell. D, Biiccinum, male, with single set of pallial organs, an. anus ; 

 au. auricle; ct. ctenidium; e. eye; exh.c. exhalant current; gon. gonad; inh.c. 

 inhalant current ; k.l. left, and k.r. right, kidney ; M. mouth ; m.s. shell muscle ; 

 mu.gl. mucous glands ; op.s. opening from mantle cavity through shell ; 

 osp. osphradium ; op.g.k. opening from gonad into kidney ; p. penis ; pb. pro- 

 boscis; p.c.m. pericardium; rhyn. rhynchocoele ; siph. siphon; ten. tentacles; 

 vd. vas deferens ; ven. ventricle ; int. intestine ; Sop. male aperture ; rm. rectum. 



have the ctenidium pointing backwards, the auricle behind the 

 ventricle and the visceral loop untwisted and symmetrical. There are 



