REPOET ON THE MONAXONIDA. xxix 



II. The Soft Tissues. 



Professor SoUas, in his Prelimiaaiy Report on the TetractiueUida of the Challenger 

 Expedition,^ has proposed to divide the entire body of a sponge into two parts. The first 

 of these, or " ecfosome," he defines as " the outer layer of the sponge, not containing 

 flagellated chambers," and the second, or " choanosome," as " the ' mark ' or ' parenchyma,' 

 distinguished by the presence of flagellated chambers." This arrangement we find to be 

 a very convenient one for practical purposes, and accordingly we shall make use of it in 

 this Report ; although applied in the first instance to Tetractinellid sponges it is equally 

 applicable to the Monaxonida, or, indeed, to most groups of sponges. So far as the de- 

 finition goes, the terms ectosome and choanosome might be used to include the skeletal 

 elements as well as the soft tissues, but, although the two primary divisions of the 

 skeleton into " dermal " and " main " do, roughly speaking, correspond to the two primary 

 divisions of the soft tissues into " ectosome " and " choanosome," we have found it much 

 the most convenient plan to treat of the skeleton all together and quite separately from 

 the soft tissues. 



(a) The Ectosome. 



Although varying very much in details of structure the ectosome invariably consists 

 of two kinds of elements, ectodermal and mesodermal. The ectodermal element always 

 consists, at any rate so far as is known, of a single layer of epidermic cells — usually flat, 

 as in EspereUa murrayi, nobis, but in one instance (according to Vosmaer^) columnar. The 

 sponge in which Dr. Vosmaer describes and figures the columnar epidermic cells is 

 Tentorium semisuberites, but this must be considered as a great exception to the general 

 rule. In our own sections of Tentorium, although they seem to be fairly well preserved, 

 we have not succeeded in detecting the columnar cells. We have nothing new to add 

 concerning the epidermis ; it is very difficult to study it satisfactorily in any but specially 

 prepared specimens. 



The mesodermal constituents of the ectosome, unlike the ectodermal, vary very much 

 in difi"erent genera both as regards quantity and character ; hence it is upon these that 

 the great diversity of the ectosome in difi"erent sponges depends. They may be very 

 small in quantity, forming only a thin layer of connective tissue immediately beneath the 

 epidermis, in which case the ectosome is reduced to a thin, in most parts pore-bearing 

 membrane (PI. XL VI. fig. 4, d.m.), for which the term dermal membrane is a very 

 convenient one,^ or they may be very strongly developed, forming in some instances 

 (Suberitidee) a thick, fibrous cortex (PI. L. fig. 1, ect.). 



1 Sci. Proc. Hoy. Dub. Soc, vol. v., pt. vi. p. 177. 



2 Sponges of the " Willem Barents" Expedition, 1880-81, p. 19., pi. iii., fig. 23. 



3 The term " dermal memhrane " is also applicahle in cases where there is a thick ectosome, of which the outermost 

 portion forms a separable membrane distinct from the remainder. 



