Lithodomus, the date-shell, one of the mus sels, bores into corals, oyster shells, etc.; the common Sasicava excavates holes in mud and soft limestone, as does Gastrochamo, Phoku and Petricola. Certain boring lamellibranchs, such as Pholas, are luminous.
A very aberrant form of bivalve mollusk is Clavagella, in which the shell is oblong, with flat valves, the left cemented,to the sides of a deep burrow. The tube is cylindrical, fringed above, and ending below in a disc, with a minute central fissure, and bordered with branching tubules. In the water ing-pot shell, the small bivalve shell is cemented to the lower end of a long Shelly tube, closed below by a perforated disc like the nose of a watering-pot..
Bivalves, in growing, usually pass through a pre-swimming larval stage called a "trocho sphere,)) resembling a top, and moved by a circlet or zone of cilia. After a while two flaps (vela) arise on each side of the mouth, form ing veliger stage; meanwhile the shells arise, and as they become larger and heavier, the young bivalve sinks to the bottom, and begins to use its afoot'for burrowing. Some bivalves, such as the fresh-water mussel Unio, go through a parasitic stage in their development.
Some bivalves arrive at maturity in a single year. The fresh-water mussels live from 10 to
12 years, while the giant clam (Tridacna gigas) probably lives from 60 years to a century.
The bivalves began to appear in the Cam brian Period; they became more frequent in the Ordovician and Silurian, but they did not abound until toward the Mesozoic Age, since the seas during the Palaeozoic Age were crowded with brachipods. Oysters date from the beginning of the Mesozoic. The genus Mucula and its allies arc very primitive forms, and nearly allied to the earliest known bivalves. Of about 15,000 known species of bivalves, two thirds (10,000) are fossil.
The class Pelecypoda (or Lamellibranchiata) is divided by the gill characters (consult Parker and Haswell's 'Zoology)) into five orders, namely: (1) Protobranchia, (2) Filibranchia, (3) Pseudo-latnellibranchia, (4) Eulamelli branchia, (5) Septibranchia; and by Dalt, from the hinge-characters, into three ordinal groups: Prionodcsmacea, Anomalodesmacea and Telco desmacea. In Neumayr's group Pakroconcha, now forming a part of the Prionodesmacea, included ncluded certain primitive types which ap pear to have given origin to certain more modern groups. For further information and the literature of the subject see Moll.uscA.