CLASS I PELECY PODA Bia)! | 
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 Saxicava arctica, Lam. Animal with closed mantle 
matters from the rectum, and some- &dges, showing foot (p), protruding from the pedal opening, 
. “ and anal (s) and branchial (s’) 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 

Fig. 591. 
