Chap. XVI. ] THE FRESH- WA TER MUSSEL. 3 1 5 



The protrusion of the foot is largely effected by the forcing of 

 blood into the lacunar spaces that abound therein. According 

 to some observers there are (1) three ciliated openings in the 

 ventral surface of the foot by means of which water can enter 

 and be admitted into the blood system, and (2) pores of com- 

 munication between the blood system and the pericardial 

 chamber. This provides for an ingenious mode of flushing the 

 renal organ and sweeping out its solid contents. The writer is, 

 however, in common with other observers, unable to satisfy 

 himself of the existence of these communicating pores, internal 

 or external. 



The Renal Organs. The renal organs or so-called organs of 

 Bo janus of the mussel are paired structures lying beneath the 

 pericardial cavity. Their relations are shown in the dissection 

 from the side, Fig. 91, and in the cross-section, Fig. 92, A. Each 

 consists of two parts : (1) a glandular part (re.), which com- 

 municates anteriorly with the pericardial cavity, and posteriorly 

 with (2) a vestibule, with delicate thin walls, communicating 

 anteriorly with the exterior by the external renal aperture 

 (e. re. a.). To show this aperture the gills must be cut away as 

 shown in Fig. 92. Just anterior to it is the external aperture of 

 the generative gland. 



Open the pericardial cavity from the dorsal aspect, divide the 

 rectum at the posterior end of the cavity, cut through the 

 auricles near their junction with the walls of the chamber, and 

 turn the rectum and heart forwards. The floor of the peri- 

 cardium will then be displayed. Beneath it in the mid-line is 

 seen the central blood sinus, and on either side the vestibule of 

 the renal organ. At its anterior end two small apertures will 

 be seen. They are the reno-pericardial apertures. Pass a guarded 

 bristle into one as far as it will go. Remove the floor of the 

 pericardial chamber and roof of the vestibule (coalescent). The 

 glandular part of the renal organ will be seen projecting like a 

 ridge into the vestibule. At the posterior end of the vestibule, 

 which does not extend so far back as the glandular part, the 

 guarded bristle, which was passed from the pericardial chamber 



