XII 



PHYLUM MOLLUSCA 



657 



external water mixes with the blood ; this is, however, certainly 

 not the case : the blood system is everywhere closed. 



The nervous system is found in its most primitive condition 

 in Nucula (Fig. 554). Instead of a single cerebro-pleural ganglion 



Hi 



FIG. 552. Sagittal section of part of enteric canal of Donax. I, lower lip ; II, intestine; III, 

 pvloric ccecum ; IV, crystal style: V. cuticle; VI, stomach ; VII, gullet; VIII, upper lip; 

 IX, mouth. (From Pelseneer's 



VII 



IX 



VI 



there are, on each side, distinct cerebral (xvi.) and pleural (I) 

 ganglia, each united by a connective with the pedal. 



The most characteristic sense-organs are the otocysts and the 

 osphradia. The otocyst auditory 

 or directive organ is always 

 placed in the foot, just behind 

 the pedal ganglion, to which it 

 is connected by a nerve : the 

 latter probably has its origin in 

 the cerebral ganglion. The 

 otocysts are developed as in- 

 volutions of the ectoderm, but, 

 except in Nucula (Fig. 554, 

 x-xii), the connection with the 

 exterior is lost, and they be- 

 come shut sacs. 



The osphradia olfactory or 

 water-testing organs are 

 patches of sensory epithelium 



situated in immediate relation with the visceral ganglia (Fig. 

 554, viii), from which they are usually said to be innervated. 

 There is, hoAvever, some reason for thinking that the osphradia 1 



VOL. T u u 



IV 



ii 



PIG. 553. Diagram of Nucula. I, anterior 



adductor ; II, foot ; III, renal aperture : 

 IV, posterior adductor; V, anus; VI, kid- 

 ney ; VII, ventricle; VIII, pericardium; 

 IX,-gonad. (From Pelseneer.) 



