656 



ZOOLOGY 



SECT 



forming a sort of cement to enclose the hard particles of the food 

 and prevent any harmful effect on the mucous membrane. 



The excretory organs occur in their simplest form in the 

 Protobranchia, in which they have the forms of cylindrical curved 



tubes or meso-nephridia (Fig. 553, vi), 

 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 com- 

 munication with its fellow of the oppo- 

 site side. In the higher forms the 

 organ becomes differentiated into a se- 

 creting portion or kidney, which becomes 

 very spongy in texture, and opens into 

 the pericardium, and a non-secretory 

 portion or bladder, which opens extern- 

 ally. Frequently there is a communi- 

 cation between the right and left 

 nephridia, and in some genera, such 

 as the Oyster, the organs become extensively branched. 



Circulatory Organs. The heart is usually perforated by 

 the rectum, but lies altogether above it in Nucula (Fig. 553, vii) 

 and some other genera ; the ordinary arrangement seems to have 



FIG. 550. Four gill-filaments of 

 Mytilus. r../'. ciliary junc- 

 tion ; ./'. filaments. (From the 



I'Hiiilii'iiliJ:. S'lti'i'il/ HtxtOi'll.) 



VIII 



VII VI 



III 



IV 



FIG. 551. Dissection of Poromya. I, anterior palp ; II, foot ; III, lamella on branchial septum ; 

 IV, valve of branchial aperture; IV, anal siphon ; V, posterior adductor ; VI, posterior re- 

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

 Pelseneer's J 



been brought about by the heart becoming folded over the intes- 

 tine and united below. In the Oyster and some other forms the 

 heart is below the rectum. Pores are often found on the surface 

 of the foot, and it has been asserted that through them the 



