428 MOLLUSCA phylum vi 



The Foot. — From the ventral surface of the visceral mass projects an 

 extensile muscular organ, known as the foot, which is capable of being pro- 

 truded beyond the margins of the valves, or entirely retracted within the 

 mantle lobes. The muscles serving to move this organ are inserted upon the 

 Shell near the adductor scars, leaving small accessory impressions. In a 

 large majority of bivalves, the foot has the familiär hatchet-shape from which 

 the class name is derived, but as an organ of locomotion, tactile use, and 

 possibly prehension, it is modified for special uses in many forms. A few 

 Pelecypods, such as Osfrea, have the foot altogether aborted, though remnants of 

 its retractor muscles exist and are attached to the valves ; and in some cases 

 {Pholadomya, Halicardia) an accessory foot-like organ, or " opisthopodium," 

 is developed at the posterior end of the visceral mass. 



In many Pelecypods the foot is provided with a gland secreting horny 

 matter which solidifies in threads after extrusion, forming a fixative tuft or 

 cable called the byssus, by which the animal adheres to extraneous objects. 

 Some sessile genera have the byssus more or less calcified, when it forms a 

 shelly plug closing a sinus or foramen in one of the valves through which it 

 passes. Many of the Pectinidae have a comb-like series of denticles (denolium) 

 on the edge of the byssal sinus, in which the byssal threads rest. In perma- 

 nently sessile forms, the byssus is usually absent. 



Gills. — On either side of the visceral mass above the foot and usually 

 extending from the palpi to the posterior adductors are the gills or denidia. 

 In a general way the ctenidium is composed of a stem carrying a nerve and 

 blood-vessel, from which on each side leaflets or slender filaments are given 

 out laterally. In the more archaic types (Nucula, Yoldia, Solemya) these gills 

 are plate-like, not organically united except by the stem, though in some 

 cases attaining a solidarity as a mass, by the interlocking of very large cilia, 

 distributed in bands or patches on the opposed surfaces of individual plates. 



These plate-like gills are termed foliobranchiate or protobranchiate. 

 According to their structure, other types of gills are intermediate between 

 these and the so-called " filibranchiate," in which the plates are elongated and 

 strap-like, and the " reticulate," in which the filaments are united by cross 

 conduits in a net-like manner. Attempts have been made to employ the 

 various types of gills as fundamental characters in Classification, but experience 

 has shown that they cannot be depended upon as the exclusive basis of any 

 systematic arrangement. 



Siphons. — When the mantle lobes are united, two posterior openings, more 



or less tubulär, are always present 

 (Fig. 640). The dorsal tube, called 

 the dorsal or anal siphon, serves for 

 the discharge of water which has 

 j, been inhaled through the ventral 

 Fifi. 640. or branchial siphon, carried to the 



Saxicava arctim Lam. Animal with closed mantle gÜls, dcprivcd of itS OXygeU and 



ges, showing foot (p), protruding from the pedal open- j-i,i j.- ^ j j.i, n j 



mg, and anal (s) and branchial (s') sTphons. Natural siL edlble particles, and then expelled. 



The anal siphon also carries efFete 

 matters from the rectum, and sometimes ova are discharged in the same 

 way. The tubes are sometimes adherent or enclosed in the same envelope, 

 and sometimes separate to their base ; in general, however, a septum or 

 partition exists between the two passages, thus avoiding the mixture of the 



