657 



I PHYLUM MOl^LUSCA 



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

 b 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 



VII VI 



IX 



VI 



V2. Sagittal section of part of enteric canal of Donax. 1, lower lip ; II, intestine ; III, 

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

 IX, mouth. (From Pelseneer's J/o//cx' /( /, .,.) 



iere 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, however, some reason for thinking that the osphradial 



VOL. I ^ tf-*^_ 



'-^B^X 



V^ ov i^V 



TY )] 



IV 



III 



Fie. 558. Diagram of Nucula. I, anterior 

 adductor ; II, foot ; III, renal aperture ; 

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

 ney ; VII, ventricle; VIII, pericardium; 

 IXygonad. (From Pelseneer.) 





