252 THE LAMELLIBRANCHIA 
the close of parasitic life, and its formation is not accompanied 
by a sensible ectodermic invagination. When the young Unio quits 
its host its evolution is not complete. The gills continue to grow 
slowly, their external plates not being developed until the third 
year, and sexual maturity is not attained until the fifth year, but 
growth continues for some time after. 
IV. BronomMics AND DISTRIBUTION. 
All the Lamellibranchia are aquatic. The great majority are 
marine, but some few families have penetrated into fresh waters. 
All the members of the class feed upon microscopic organisms, 
chiefly Diatomaceae and other low forms of plant life. Only the 
Septibranchia and some other abyssal forms are truly carnivorous. 
In general, the Lamellibranchs are burrowing forms, living 
half-buried in muddy or sandy bottoms, and in this case their 
plane of symmetry is vertical. But many forms are completely 
sedentary and are fixed by the byssus, or in a more definitive 
manner, by the shell itself, as is the case in Spondylus, Ostraea, 
Aetheria, Myochama, ete: In these genera the plane of symmetry 
becomes horizontal, and the animal usually lies on the right side, 
e.g. Pinna, Hinnites, Spondylus, Plicatula, Anomia, and the Rudistae ; 
more rarely on the left side as in Ostraca, Requienia, and Chama 
generally. Some Lamellibranchs live in holes which they excavate 
either in wood, as in the case of Zeredo, or in stone, as Lithodomus, 
Saxicava, Pholas, Clavagella, ete., or even in the shells of other 
Molluses. Lithodomus is only found in caleareous rocks, and bores 
its hole by the aid of the acid secretion of glands situated in the 
antero-dorsal and postero-dorsal regions of the mantle. 
Some Lamellibranchs, such as Lima, are nidamentous, and 
construct a nest by means of the byssus. Lima hians builds its 
nest in the space of three weeks, and may afterwards return and 
reconstruct another from it. Modiolaria marmorata and Entodesma 
cuncatum pass their existence deeply buried in the tests of 
Ascidians, and Vulsella lives in a similar manner in sponges ; but 
the few commensalistic or parasitic forms generally live on or in 
Echinoderms: thus Montacuta lives on Spatangids, Scioberetia in 
the incubatory pouch of an Asterid, Hntovalva in the oesophagus of 
a Synapta. On the other hand, Hphippodonta is commensal with a 
prawn, and certain species of Lepton with Gebia. 
Only a few species are very active: Tellina, Yoldia, etc., execute 
leaping movements by forcibly contracting the foot ; Lasaea, Cyclas, 
etc., crawl on immersed bodies or on the surface of the water ; 
other forms, notably the Pectinidae and Limidae, swim by rapidly 
opening and closing the valves of the shell; and some elongated 
forms in which the mantle edges are fused for a considerable 
