150 A. EF. Verrill—The Bermuda Islands; Geology. 
much larger were it not for the undermining of their bases by bor- 
ing sponges, mollusks, annelids, ete. 
Among the most common and important of the borers are bivalve 
mollusks of the genus Lithophaga. One of these, a dark brown or 
black species, about 2 inches long (L. nigra), is very common in 
the bases of large corals. Other species with the same habits are 
L. appendiculata (fig. 48, 43a) and JL, bisuleata. Other boring 
bivalves common here are Gastrochena rostrata and Coralliophaga 
coralliophaga.* 
Several species of annelids are constantly found in irregular 
burrows and tubes in the dead or partly dead corals, but it is uncer- 
tain, in most cases, whether they make the burrows or simply occupy 
Figure 436, a-d.—Physcosoma varians; from corals, showing different color 
varieties and states of contraction. Phot. by A. H. Verrill. 
those made by sponges and mollusks. Several large species of Jeo- 
dice, Marphysa, Nicidion, etc., are particularly common and with 
other forms found in corals will be referred to later, in discussing 
the life of the reefs. See chapter 30. 
Several species of Gephyreans also occur. One of these, Physco- 
soma varians (fig. 436), which is very common, seems to be a true 
borer, but may be only an intruder.t 
Certain species of siliceous sponges are among the most destrue- 
tive of the boring animals that attack corals and shells. They make 
* See these Trans., vol. x, pl. xiii, figs. 9, 10. 
+ See these Trans., vol. x, p. 669; Verrill, Zodlogy of Bermuda, art. 9, p. 669, 
1900. 
