REPORT ON THE TETRACTINELLTDA. 
XXV 
This result leads us to a further critical examination of the intercortical cavities of 
the fully grown sponges ; and from it I am inclined to think that the following differences 
will be found to obtain generally, though perhaps not universally. 
In Stelletta, Geodia, and their allies the intercortical cavities are separate and 
independent chones, which when traced centripetally are frequently found to open into a 
common subcortical crypt (PI. XXVII. fig. 13). In Craniella the intercortical cavities 
form large subdermal chambers, which when traced centripetally are found to lead each 
into several separate and independent incurrent canals. The relations of the two types 
to each other are shown on the accompanying diagram (Fig. VII. ). 
The Canal-System. 
The spongophare or choanosome as the case may be is folded on very different plans 
in different sponges, and these are expressed in the different disposition of the incurrent 
and excurrent canals. The arrangement of the main canals has been spoken of by Zittel 
as the type of canal-system, a convenient expression which has been unfortunately 
rendered ambiguous by other writers, since they have used it to denote the nature of the 
relations of the flagellated chambers to the ultimate branches of the incurrent and 
excurrent canals. As, however, Zittel’s use of the term seems the more appropriate, and 
to have priority, I shall retain it in his sense, and replace it as used in the other sense by 
the term “type of chamber-system.” 
The details of the canal system can seldom be clearly traced, since in all but the 
youngest or smallest sponges they are obscured by metamorphosis and concrescence. 
No doubt a study of young examples of the larger sponges, in various stages of growth, 
will enable us to unravel many of the complexities presented by the adult, but this will 
be a laborious task, and one for which the Challenger material is not adequate. The study 
of this subject involves also a knowledge of the manner in which the adult increases 
in size, and regarding this we are at present ignorant ; is growth merely additive, 
occurring chiefly at the superficies, or is it to any great extent also interstitial ? So 
far as increase in size is generally concerned, I am for many considerations strongly 
inclined myself to regard it as mainly exogenous, occurring chiefly in a zone immediately 
beneath the ectosome ; buf that interstitial growth does also occur is certain, as obser- 
vations made on the distribution of the spicules in the cloacal tube of Tribrachium 
schmidtii prove (see p. 157), the cladomes of these spicules becoming more removed from 
each other as they are traced from the origin of the tube towards its termination, and. this 
can only be due to interstitial growth. 
As our knowledge is so evidently limited, this chapter must be regarded rather as 
pointing out questions for investigation than a summary of positive results. 
(ZOOL. CHALL. EXP. — PART LXIII. 1888 .) 
Err d 
