620 MOLLUSCA— CLASS II.—PELECYPODA. 



and the interlamellar junctions become solid connections, so that the whole 

 structure presents a spongy appearance, whilst the reflected ends of the 

 Ulaments, uniting with the walls of the mantle and foot, subdivide the pallial 

 chamber (0, Fig. 7). One further development remains to be mentioned, in 

 which the gills have become converted into a sort of party wall. The follow- 

 ing diagrammatic drawings will make these various stages more clear (Fig. 7). 



The muscular foot is a prominent feature in most bivalves, lying in the 

 middle line towards the front of the body. It is actuated by special muscles, 

 and is principally of service in forcing a way for the creature into the sand 

 or mud; in a few it is a creeping organ; whilst the cockles, by its means, 

 take extraordinary leaps. In some forms it is greatly reduced, or altogether 

 wanting. Towards the back of the foot is situated the apparatus for 

 spinning the horny threads by which many bivalves anchor themselves 

 to stones, seaweed, etc. The common mussel is a familiar example, 

 and the bunch of threads is known as the byssns. The animal generally 

 has the power of rejecting its byssus at any time and spinning a fresh 

 one. 



The mouth ia situited a little behind and beneath the front adductor 

 muscle; it is unarmed, neither jaw nor radula being present. It ia flanked 

 on either side by a pair of twin, triangular lobes, the labial palps, which are 

 in a line with, and in front of, the gills. Their function apparently is to 

 collect, and possibly to taste, the food before it passes into the mouth. A 

 short cesophagus leads to the stomach at the base of the foot, whence the 

 intestine, more or less coiled, leads off, and, after passing through the peri- 

 cardium, termiuates beneath the hinder adductor muscle, at the base of the 

 exhalent siphon. 



The heart, in addition to the ventricle, which frequently encircles the 

 alimentary canal, has usually two auricles. The nervous system is sym- 

 metrical, and consists typically of three pairs of ganglia : — The cerebro- 

 pleural, resulting from the fusion of the cerebral and pleural ganglia, lying 

 above the oesophagus ; the pedal ganglia, which are situated in the foot, near 

 its base ; and the visceral ganglia, placed close to the posterior adductor 

 muscle. These ganglia are connected by nerve cords, and supply nerve 

 fibres to the various organs. The remaining anatomical features call for no 

 comment here. 



The classification of the Peleeypoda is a most difficult and vexed question ; 

 but that founded on the structure of the gills, as explained above, will be 

 employed here. The five orders thus created are ; — 



I. PROTOBRANCHIATA. 



II. FILIBRANCHIATA. 



III. PSEUDOLAMELLIBEANCHIAIA. 



IV. BULAMELLIBRANCHIATA. 

 V. SEPTIBRANCHIATA. 



Order I. — Protobranchiata 

 Contains the most primitive forms of Peleeypoda. The gills are folio- 

 branchiate in type, and the mantle margins generally open. The foot has a 

 creeping disc on its ventral surface. 



Two families are included in this order — the Nuculid.e and the Soleno- 

 MYii)^. To the former belong the little nutshells (Nucula), small triangular 

 shells of a plain brown exterior and pearly interior, provided with a series 

 of fine transverse teeth in the hinge, calling to mind a comb, with the re- 



