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



(b) The Choanosome. 



According to Schulze -^ the ectoderm of the larval sponge furnishes not only the 

 epithelium clothing the surface of the sponge, but also the epithelial lining of the canal 

 system from the pores on the surface to the inhalent apertures of the flagellated 

 chambers ; while the endoderm furnishes the collared cells and also the epithelial 

 lining of the canal system, from the exhalent apertures of the flagellated chambers to the 

 margin of the oseulum ; the remainder of the sponge is mesodermal. 



If this \dew be correct, all three germinal layers, viz., ectoderm, mesoderm and 

 endoderm, take part in the formation of the choanosome. 



Nevertheless the choanosome presents us with much less variety in histological 

 structure than does the ectosome ; a fact which is probably to be accounted for by the 

 circumstance that it does not come into such direct contact with external conditions, 

 and is hence less subject to modification. All the diS'erent histological elements (viz., 

 stellate, amoeboid, flagellated, pavement and fibrous cells) which take part in the formation 

 of the choanosome in the Monaxonida, are, with the exception of the flagellated cells, 

 found also in the ectosome, and we may, accordingly, dismiss this part of our subject very 

 briefly. The extent to which the ground substance, in which the flagellated chambers are 

 embedded, is developed varies much. In the Clavulina there is, at any rate as a 

 general rule, very little indeed, and the flagellated chambers are packed very closely 

 together (vide PI. L. fig. lb). In the Halichondrina (e.g., Esperella murrayi, PI. XLVIII. 

 fig. 2d, and Axinella (f) paradoxa, PI. XLIX. fig. 2) there is commonly more, and 

 the chambers lie further apart from one another. It is moreover sometimes stated that 

 there is a distinction between these two groups in the character of this fundamental 

 tissue ; that in the Clavulina it is finely granular, while in the Halichondrina it is 

 clearer and more gelatinous. That there is some difi"erence of this nature there can be no 

 doubt, but it has probably also been somewhat confounded with the diff"erence already 

 mentioned, viz., the much smaller total amount of ground substance present in the 

 Clavulina as compared with the Halichondrina. 



"We have, in short, no new facts to add concerning the histological structure 

 of the choanosome, except with regard to the fibrous tissue accompanying the bands 

 of spiculo-fibre, and this has already been fully dealt with in treating of the 

 skeleton. 



Special Structures. 



We may conveniently describe in this place certain very remarkable and quite unique 

 structures present in an abyssal sponge, Cladorhiza (?) tridentata, nobis. The shape of 



I Zeitschr. f. wiss. Zool, Bd. xxxiv. p. 438. 



