THE ANATOMY OF THE MADREPORARIA. 413 
chamber with its Sipunculid tenant. Some species of all 
three genera are known to derive the curvature of the chamber 
from settlement on a heliciform shell, as the latter is in some 
specimens only partially obscured by the overgrowth of coral ; 
for instance, Heteropsammia analoga on a Turritella 
(British Museum), Heterocyathus parasiticus on Nassa 
and Cerithium (Semper), Stephanoseris lamellosa on at 
least seven genera! (British Museum). Further, in the case of 
the last-named species, a complete series might be arranged 
from the specimens in the National Collection, showing how, 
as growth proceeds, the characteristic outline is acquired. 
The embryo coral settles near the mouth of the shell (in this 
instance a Nassa), round which it gradually extends; at the 
same time, the original mouth being unnecessarily large for 
the Sipunculid, the latter roofs it over with a deposit of (?) 
lime (cf. Forbes’ ‘ British Starfishes,’ p. 252. Van Voorst, 
1841, 8vo.) excepting only for one large pore in the centre, 
and a lesser one higher up, which, when the shell has been 
covered, appears to lie just at the base of the theca, and which 
is continuous with, or a prolongation of, the anterior canal of 
the lip. Growth of the Stephanoseris obscures the shell and 
this artificial roof more and more, gradually producing the 
characteristic outline. As increase of bulk proceeds, the 
Sipunculid (?) lines the two pores with a material which resem- 
bles a Serpula tube. The lesser pore? in S. lamellosa 
appears to be homologous with the tiny pores placed either 
on the base of the whole structure (S. philippinensis), or 
round the bases of the thecze (Heteropsammia). The lesser 
pore may coexist with tiny pores on the base (S. exarata). 
To these further reference will be made below. 
Again, a large number of Gephyrea normally inhabit helici- 
1 The genera represented in the British Museum on which this species has 
settled are Mitra (5 sp.), Drillia, Columbella, Strombus, Pleurotoma, Ceri- 
thium, Nassa (2 sp.). For this determination, and for many other courtesies, 
I am indebted to the gentlemen in charge of various departments in the 
Museum, and especially to Professor F. J. Bell. 
2 The specimens of Heterocyathus which I have examined possess a similar 
pore (cf. Semper, ‘ Animal Life,’ p. 336). 
