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. BIONOMICS 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, etc. In these genera the plane of symmetry 

 becomes horizontal, and the animal usually lies on the right side, 

 e.g. Pinna, Hinnites, Spondylus, Plicatula, Anemia, and the Rudistae ; 

 more rarely on the left side as in Ostraea, Eequienia, and Cliama 

 generally. Some Lamellibranchs live in holes which they excavate 

 either in wood, as in the case of Teredo, or in stone, as Lithodomus, 

 Saxicava, Pholas, Clavagella, etc., or even in the shells of other 

 Molluscs. Lithodomus is only found in calcareous 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 Mans builds its 

 nest in the space of three weeks, and may afterwards return and 

 reconstruct another from it. Modiolaria marmorata and Entodesma 

 cuneatum 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, Entovalva in the oesophagus of 

 a Synapta. On the other hand, Ephippodonta 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 



