14 THE VOYAGE OF H.M.S. CHALLENGER. 



the inner entodermal epithelium of the canal-system. Comparing the descriptions which 

 are given of the former in many different sponges, I am more inclined to regard it as a 

 superficial epithelial differentiation of the mesoderm, with which it remains in closest 

 connection. The fact that the main mass of the sponge-body (or the so-called 

 mesoderm) belongs histologically to the connective tissue, is not in contradiction with 

 its conception as exoderm. We know that the peculiar mantle of the Ascidise is a 

 voluminous and most remarkable product of the exoderm ; it is a true connective 

 tissue in histological respect, but a true exodermal (not mesodermal !) production 

 in genetical respect. The cells, which are scattered in the connective ground-mass of 

 the Ascidian tunic, and which are derived from the epidermis (!), are often arranged on 

 the outer surface of the thickened tunic in the form of an outer simple layer of pave- 

 ment-cells (a quasi-secondary epidermis). This may be compared to the surface- 

 epithelium of the sponges. The histological comparison of the tunic of the Ascidise 

 with the so-called mesoderm of the sponges seems to be justified, especially as the further 

 differentiation of both of them is often very similar. 



Entoderm (Canal-Epithelium). — In opposition to the exodermal surface-epitheliuin, 

 which we may regard only as a secondary superficial production of the primary outer 

 cell-layer, the entodermal epithelium of the canal-system is independent from the 

 beginning, a self-subsistent inner group of cells, which is separated already in the 

 Gastrula from the different exodermal group (the fundament of the later mesoderm and 

 the secondary exoderm). This entodermal or gastral epithelium seems to have in the 

 Keratosa — and similarly in the Calcarea — two modes of development. It remains as a 

 single continuous layer of flagellated cells through all the cavities of the canal-system 

 in the Ammoconidas (PI. VIII.), closely agreeing with the Ascouidse (Asconal-type). It 

 is differentiated into two very different portions in all the other Keratosa, the canal- 

 system of which is developed on the Leuconal^pe (as in the Leuconidse). The 

 flagellated epithelium remains here restricted to the flagello-chambers, whilst the ento- 

 derm in all the other parts of the canal-system is a simple flat pavement-epithelium. 



Mesoderm. — The main mass of the sponge-body, which is usually now called the 

 mesoderm, and which we derive from the original primitive exoderm, exhibits in the 

 various sponges, as is well known, an infinite variety of detailed structure, mainly in the 

 production of the skeleton. Eegarded histologically, the mesoderm is always a kind of 

 connective tissue or malthar tissue, 1 and exhibits similar manifold differentiations to those 

 of the higher Metazoa. It is a relatively thin lamellar plate in the Ammoconidse (as 

 also in the Asconidaj), whilst it becomes massive and voluminous in the other Keratosa 

 (as in the majority of sponges). We distinguish in the malthar tissue of the Keratosa 

 (as in the various connectiva of other sponges) the following constituents : — 



1 On the conception of malthar tissue (Maltha) and its distinction from fulcral tissue (Fulcrum), compare the 

 Report on the Deep-sea Medusae (Zool. Chall. Exp., pt. xii. vol. iv. § 50, p. xxxi). 



