STRUCTURE AND PHYSIOLOGY OF THE MOLLUSCA. 33 



current, setting in through the (branchial) orifice, escaping through 

 the meshes of the net, and passing out by the anal orifice of the 

 outer tunics. The regularity of this current is interrupted only 

 by spasmodic contractions of the mantle, occurring at irregular 

 intervals, by which the creature spirts out water from both orifices, 

 and thus clears its cavity of such accumulated particles as are 

 rejected by the mouth ; and too large to escape through the 

 branchial pores. In the salpians, these contractions are ryth- 

 mical, and have the effect of propelling them backwards. In the 

 ordinary bivalves, the gills form two membranous plates on each 

 side of the body ; the muscu- 

 lar mantle is still sometimes 

 united, forming a chamber 

 with two orifices, into one 

 of which the water flows, 

 whilst it escapes from the 

 other ; there is a third open- 

 ing in front, for the foot, but 

 this in no wise influences the 

 branchial circulation. Some- ^ 18 - Trigonia pectinata* 

 times the orifices are drawn out into long tubes, or siphons, es- 

 pecially in those shell-fish which burrow in sand. (Figs. 19 

 and 7.) 



Fig. 19. Bivalve wth long siphons.^ 



stances, with the aid of a microscope ; but the currents they cause are easily 

 made perceptible by dropping fine sand into the water over them. 



* Trigonia pectinata, Lam. (original). Brought from Australia by the 

 late Captain Owen Stanley. The gills are seen in the centre through the 

 transparent mantle, o, mouth j / 1, labial tentacles ; f t foot ; v, vent. 



f Fig. 19. Psammobia vespertina, Chemn. after Poli, reduced one half. 

 The arrows indicate the direction of the current, r $, respiratory siphon. 

 t *, excurrent siphon. 



C 3 



