CLASS l PELECYPODA 3ft] 



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 familiar 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 

 mollusks, such as Ostrea, 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 (ctenolium) 

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

 manently 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 ctenidia. 

 .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 tubular, are always present (Fig. 591). The dorsal tube, called the 

 dorsal or anal siphon, serves for the 

 discharge of water which has been 

 inhaled through the ventral or 

 branchial siphon, carried to the 

 gills, deprived of its oxygen and 

 edible particles, and then expelled. 

 The anal siphon also carries effete Sa ^ cnm ffrc ,,.,, ^.4^ with oloswl lnailtlM 



matters from the rectum, and some- e(1 ^ es ' showing foot oo, raotrndtag from the p.-iai opening, 



, . , ,. , and aiuu m and branchial (/) siphons. Natural .size. 



times 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 

 two currents. The siphonal septum is frequently carried forward internally, 

 or supplemented by a junction of the gills in such manner as to form a 



