- The genus Monobrachium is a colonyforming hydroid which grows upon the shells of bivalve molluscs, each polyp having but a single tentacle.
C, After becoming attached, side B, Cypris-larva with a bivalve views.
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.
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.
It may assume the form of a bivalve shell entirely enclosing the body and limbs, as in many 6, maxilla (second maxilla).
The sorus has a somewhat elongated receptacle, on which the sporangia arise basipetally; the indusium may be cup-shaped, bivalve or wanting.