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



S'pongin-Fibrittee. — The fibrillas characteristic of the Stannomidse are imbedded in 

 the clear hyaline maltha or the connective ground-mass, and exhibit the same physical 

 and chemical peculiarities as the well-known corneous fibres of the common Keratosa ; 

 they consist, therefore, of spongin (or spongiolin). Usually they are simple cylindrical 

 filaments, rarely a little branched, never anastomosing or reticular. Their colour is 

 yellow, sometimes light brownish (PI. II. figs. 1-3, f; PI. III. fig. 9). 



Size of the Fibrillse. — The fibrillse are in general very long, but difficult to determine, 

 since it is usually impossible to isolate them for their whole length ; in some macerated 

 specimens, however, I was able to separate fibrillse 2 to 5 mm. in length, and in one 

 case even a thread 11 mm. in length. I suppose that they often really attain a length 

 of some centimetres or more; perhaps often (or even constantly?) a great part of the 

 fibrillse run uninterruptedly from the base of the sponge to its periphery. Their 

 thickness is usually equal throughout their whole length, viz., 0"001 to 0"004 mm. on 

 an average, but sometimes the thicker fibrillse attain a diameter of O'Ol to 0"02 mm., 

 whilst the thinnest threads are only O'OOOl to - 0005, or even less. In the majority 

 of the Stannomidse the thickness of the fibrillse varies very little, and is nearly constant 

 iii one and the same specimen. 



Arrangement of the Fibrillse. — The arrangement of the spongin-fibrillse in the body 

 of the Stannomidse is rather variable, and seems to depend often upon the mode of 

 growth and the development of the pseudo-skeleton and of the symbiotic Hydroids. 

 Often all the fibrillse run isolated, irregularly interwoven in all directions. But usually 

 the fibrillse are aggregated densely in bundles, connected by a minimum quantity of 

 maltha. The smaller bundles are composed of four to eight, the larger of ten to 

 twenty or more parallel fibrillse. When the bundles branch, a part of the unbranched 

 fibrillse separates from the rest and passes into the branch, similar to the nervous 

 primitive fibres in a branching nerve. The fibrillse themselves do not usually branch, 

 but in some of the Stannomidse, and especially in those in which the pseudo-skeleton 

 is composed of Globigerina ooze, the thicker fibrillse branch frequently. The branches 

 are sometimes of equal, at other times of unequal, thickness ; they never anastomose in 

 the true Stannomidse. As soon as the neighbouring branched fibrillse anastomose and 

 form a network, the Stannomidse pass over into Spongelidse. So Stannophyllum (from 

 Station 271) passes over into Psammophyllum (Stations 241 and 244). 



Structure of the Fibrillse. — The thinnest fibrillse appear under the microscope, even 

 with the highest powers, perfectly structureless ; but in the thicker threads, mainly the 

 thickest forms (0 - 01 to 0'02 mm. in diameter), may be clearly distinguished a central 

 medullary substance or an axial thread and a peripheral cortical substance ; the latter 

 is usually also in the thickest threads much broader than the former, but in some of 

 the Stannomidse distinguished by rather thick fibrillse the axial thread is twice as 

 broad as the surrounding cortical tube. In some macerated specimens the axial canal 



