EEPOET ON THE TETEACTINELLIDA. 
XXI 
prosopyle becomes continued into a comparatively long prosodus. This, which may be 
called the diplodal type of chamber-system, is rarely met with among the Tetractinellida, 
or indeed in the Parazoa generally. Schulze ^ describes it in Cortidum candelabrum, 
0. Schmidt, and I have met with it in Thrombus challengeri and Azorica jpfeifferse. It 
appears to correspond to Vosmaer’s fourth type of chamber-system.^ 
Since the progressive change from one type of chamber to another is associated with 
a change in the characters of the mesoderm and its increased development, we might 
expect some marked differences, evident to the unaided senses, in the general character 
of sponges distinguished by different types of chamber-system. Such differences 
certainly do exist, but they are of little value to the investigator. On the whole sponges 
with eurypylous chambers are less dense, and those with diplodal chambers denser than 
those with aphodal chambers. Sponges with diplodal chambers are usually remarkably 
compact and “ fleshy.” 
The characters of the chamber-systeA are most easily determined in the eurypylous 
type ; in sponges belonging to it the chambers are sometimes clearly displayed in thick 
slices cut free hand from a spirit specimen, with no further preparation than staining 
and mounting in glycerine. 
The Ectosome. 
The change in the character of the canal-system is usually but not invariably 
{Thrombus and Azorica are exceptions) accompanied by considerable modifications of the 
ectosome. These appear to be of different nature in different sponges, and even when 
the final products are structurally similar, they may have been differently evolved. 
Embryological evidence as to the precise history of the ectosome is however scanty, and 
consequently the following explanation must be regarded as to some extent hypothetical. 
In sponges with eurypylous chambers the ectosome never attains any high degree 
of differentiation. It consists, as shown in the diagram (Fig. III., p. xv), of an in- 
vesting membrane composed of a thin layer of mesoderm bounded on both sides by 
ectodermal epithelium, together with the metamorphosed ends of the excurrent lobes, 
which place the outer membrane in continuity with the rest of the sponge ; these meta- 
morphosed ends we shall speak of as the pillars of the subdermal cavities. The external 
or dermal membrane may attain a thickness of nearly a millimetre and sometimes 
presents considerable histological differentiation ; but the subdermal cavities are never 
completely differentiated from the incurrent canals, and always communicate directly 
with more or fewer of the flagellated chambers (PI. I. figs. 12, 27 ; PI. VII. fig. 4 ; 
PI. VIII. fig. 9). 
^ Schulze, Zeitschr.f. wiss. Zool., Bd. xxxv., 1881. 
^ Vosmaer, Bronn’s Klassen u. Ordnung, d. Thierreichs, Porifera, p. 131, fig. 7. 
