SECT. XII 



PHYLUM MOLLUSCA 



681 



delicate, semi-transparent substance (m.) is seen the edge of the 

 mantle or pallium. The mantle really consists of separate halves 

 or lobes corresponding with the valves of the shell, but in the 

 position of rest the two lobes are so closely approximated as to 

 appear simply like a membrane uniting the valves. At one end, 

 however, the mantle projects between the valves in the form of 

 two short tubes, one (ex. sph.) smooth-walled, the other (in. sph) 

 beset with delicate processes or fimbriw. By diffusing particles of 

 carmine or indigo in the water it can be seen that a current is 

 always passing in at the fimbriated tube, hence called the inhalant 

 siphon, and out at the smooth or exhalant siphon. Frequently a 

 semi-transparent, tongue-like body (ft.) is protruded between the 

 valves at the opposite side from the hinge and at the end furthest 

 from the siphons : this is the foot, by its means the animal is able 

 slowly to plough its way through the sand or mud. When the 



(t pa 



FIG. 562. Anodonta cygnea. The entire animal. A, from the left side ; -B, from the 

 posterior end ; d. p. a. dorsal pallial aperture ; ex. xph. exhalant siphon ; ft. foot ; in. sph. 

 inhalant siphon ; Ifj. ligament ; m. mantle ; um. umbo. (After Howes.) 



Mussel is irritated the foot and siphons are withdrawn and the 

 valves tightly closed. In a dead animal, on the other hand, the 

 shell always gapes, and it can then be seen that each valve is 

 lined by the corresponding lobe of the mantle, and that the 

 exhalant siphon is formed by the union of the lobes above and below 

 it and is thus an actual tube ; but that the boundary of the 

 inhalant siphon facing the gape of the shell is simply formed by the 

 approximation of the mantle-lobes, so that this tube is a tem- 

 porary one. 



The hinge of the shell is dorsal, the gape ventral, the end 

 bearing the siphons posterior, the end from which the foot is 

 protruded anterior : hence the valves and mantle-lobes are re*- 

 spectively right and left. 



In a dead and gaping Mussel the general disposition of the 

 parts of the animal is readily seen. The main part of the body 



