XII 



PHYLUM MOLLUSCA 



05 



anterior adductor. It is usually bounded by two pairs of labial 

 palps which sometimes attain an immense size (Fig. 58H); there is 

 never any trace of jaws or other masticatory apparatus. The 

 convolutions of the intestine are some- 

 times very complex. The crystalline style 

 either lies freely in the stomach and 

 anterior part of the intestine, or is con- 

 tained in a csecal pouch of the stomach 

 (Fig. 590), which may be prolonged into 

 one of the lobes of the mantle. The 

 anterior end of the style, which projects 

 into the stomach, appears to be slowly 

 dissolved by the digestive juice, forming 

 a sort of cement to enclose the hard 

 particles of the food and prevent any 

 harmful effect on the mucous membrane. 

 It is possible also that the dissolved 

 substance of the style may play the part 

 of a digestive secretion, since it appears 



to contain a substance of the nature of a digestive ferment 

 capable of acting upon starchy matters. 



The excretory organs occur in their simplest form in the 

 Protobranchia, in which they have the form of cylindrical curved 



FIG. 58S. Four gill-filaments of 

 IXEytilus. c.j. ciliary junc- 

 tion ; .;'. filaments. (From the 

 Cambridge Natural History.) 



VIII 



VII VI 



IV 



Fio. 589. Dissection of Poromya. I, anterior palp ; II, foot ; III, lamella on branchial septum ; 

 IV, valve of branchial aperture ; IV, anal siphon ; V, posterior aclductor ; VI, posterior i - e- 

 tractor of foot ; VII, heart ; VIII, ovary ; IX, branchial septum ; X, anterior adductor. (Fn>m 

 Pelseneer.) 



tubes or nephridia, opening at one end into the pericardium 

 and at the other on to the exterior ; the whole nephridium 

 is lined with glandular epithelium, and has no communication with 

 its fellow of the opposite side. In the higher forms the organ 



