- The genus Monobrachium is a colonyforming hydroid which grows upon the shells of bivalve molluscs, each polyp having but a single tentacle.
The Acephala, or Lamellibranchiata, are commonly known as bivalve shell-fish.
The males are usually more brilliantly coloured than the females, and guard the eggs, which are often placed in a sort of nest made of the shell of some bivalve or of the carapace of a crab, with the convexity turned upwards and FIG.
An example of the latter is seen in a common little freshwater bivalve, the Pisidium pusillum, which has been studied by Lankester.
A two-edged weapon, of which the blade is of sharks' teeth, and a defensive armour of braided sennit, are also peculiar to the islands; a large adze, made of the shell of the Tridacna gigas (the largest bivalve known), was formerly used in the Carolines, probably by the old builder race.
The byssus is a collection of horny threads by which the sea mussel (like many other Lamellibranch or bivalve molluscs) fixes itself to stones, rocks or submerged wood, but is not a permanent means of attachment, since it can be discarded by the animal, which, after a certain amount of locomotion, again fixes itself by new secretion of byssus from the foot.
It may assume the form of a bivalve shell entirely enclosing the body and limbs, as in many 6, maxilla (second maxilla).
C, After becoming attached, side B, Cypris-larva with a bivalve views.
The sorus has a somewhat elongated receptacle, on which the sporangia arise basipetally; the indusium may be cup-shaped, bivalve or wanting.
The Ostracoda have the body enclosed in a bivalve shell-covering, and normally unsegmented.