REPORT ON THE TETRACTINELLIDA. 
XV 
pari passu with the increase by growth of the sponge. In this way the simple structure 
of Oscarella lobidaris, 0. Schmidt, and Placina monoloplia, F. E. Schulze, may be 
explained. 
The inner sinuses of the folds or lobes of the spongophare are continuous with the 
original cavity of the paragaster ; they and the remains of the paragaster are now known 
as “ excurrent ” canals ; the outer sinuses, known as “ incurrent ” canals, are open to the 
exterior ; their open ends are known as “ pores.” 
Eurypylous Chamber-System. — The flagellated chambers continue as in the Ehagon to 
communicate directly with the excurrent canals through the apopyles, which are not con- 
tinued into special tubes ; and in all such cases, where several chambers open directly 
into a common excurrent canal, we shall speak of the chamber-system as “ eurypylous.” 
The extreme simplicity which characterises the folding of the spongophare in 
Placina is not to be met with in any other genus of Tetractinellida, but it is closely 
approached in the case of certain species of Tetilla and Thenea. In Tetilla pedifera, 
Sollas (PI. XLI. flg. 7), the spongophare retains the primitive simplicity of structure 
Fig. III. — Diagram of a transverse section through the outer region of Tetilla pedifera. E, ectosome ; 
C, choanosome ; e, excurrent canal ; i, incurrent canal ; p, pores. 
it possesses in the Ehagon almost throughout the whole sponge ; but it is folded to a 
far greater extent and in a far less regular manner than in Placina monolopha. This 
indeed follows as a necessary consequence of its immensely greater size, particularly in 
thickness. Furthermore, concrescence occurs wherever the folds are brought in contact, 
and this makes the nature of the folding more difficult to analyse. 
Ectosome and Choanosome. — An additional and important modification is to be found 
in the presence of an investing membrane which surrounds the whole of the free surface of 
the sponge ; it roofs over the incurrent canals which now communicate with the exterior 
through groups of small apertures which traverse it, and which are called “ pores,” 
though evidently not quite homologous with the apertures so designated in Placina ; and 
it is confluent with the lobes of the folded spongophare where these lie immediately 
beneath it. 
This investing skin may be distinguished as the ectosome, the rest of the sponge as 
the choanosome, the latter name framed in allusion to its being the region to which the 
choanocytes lining the flagellated chambers are restricted. 
