LAMELLIBKANCHIATA. , 505 



distributes branches to the mantle, to the gills, and sends forwards 

 two long filaments, parallel with each other, one on each side of the 

 visceral mass, to the sides of the mouth, where they form a pair of 

 small ganglions, communicating with each other Ij a transverse 

 branch above the mouth, supplying the labial processes, and forming 

 a second feeble communicating arch beneath the mouth, from which 

 the gastric nerves are continued. 



After a few examinations, guided by the foregoing sketch, of the 

 anatomy of the most simple bivalve, the student will be prepared to 

 comprehend and avail himself of the further details of the modifica- 

 tions of the Lamellibranchiate type of organisation. 



In most dimyary bivalves the foot is an organ of locomotion. To 

 some which rise to the surfsice of the water, it acts, by its expansion, 

 as a float ; to others it serves, by its bent form, as an instrument to 

 drag them along the sands ; to a third family it is a burrowing organ ; 

 to many it aids in the execution of short leaps. In the piddocks 

 {Pholas) two strong muscles are sent from the foot to be attached to 

 the rough spatulate terminations of the bent processes under the beaks 

 of the valves ; these serve as fulcra in the excavating actions of the foot. 



We may generally observe in relation with the greater develop- 

 ment and more active functions of the foot, a corresponding com- 

 plexity of the respiratory system. This is effected by the super- 

 addition of accessory organs in the form of tubular prolongations 

 of certain parts of the margin of the mantle, which are provided 

 with a special development of muscular fibres : these are called 

 "siphons" {ib.g,h), and have for their retraction special muscles 

 attached to the valves of the shell : in the Pholas crispaia there is a 

 third small accessory gill on each side. 



In the Mediterranean bivalve called Cytherea chione (Jig. 189) we 

 have an example of one of these highest organised of the acephalous 

 MoUusca, in which the characters of its grade of structure are in- 

 structively impressed upon the valves of the shell. The muscular 

 fibres (a, a) which retract the margins of the mantle have their fixed 

 point within the margin of the shell, and leave there a linear im- 

 pression at a a : the closing shell-muscles leave deeper im- 

 pressions ; b is the anterior adductor, and i' its corresponding 

 impression ; c is the posterior adductor, and c' its impression ; d 

 is the retractor of the siphons, and d' its impression : e shows 

 the transverse fibres of the foot, which, by their contraction, 

 lengthen the organ, and cause it to protrude : / marks the longi- 

 tudinal fibres or retractors of the foot : their origin leaves a small 

 roundish impression near the back part of the hinge of the sliell. 

 When the siphons are present, they are always two in number, cor- 



