PORIFERA. I. 



spread it is distinct and easily separable, while in forms with small siibdermal cavities it may be 

 little or not at all marked off inwardly. The term Ectosome, which is now most frequenth' used 

 of this part, I have not employed, as the question is not here of a structure of a definite morpho- 

 logical value, and the term dermal membrane is therefore only used as a descriptive term. The dermal 

 membrane may be a thin membrane without spicules, but may also be provided with spicules variously 

 grouped, and in the latter case we may speak of a dermal skeleton, but this .skeleton is frequently 

 not distinctly marked off from the other skeleton of the sponge, being only its outer part. The 

 dermal membrane is frequently, especially when it is a thin membrane without spicules, supported on 

 spicules projecting from the skeleton; in such cases I have emploved the descriptive expression that 

 the dermal membrane is pierced by spicules and the surface shaggy. In the undamaged sponge a 

 real piercing hardly takes place, but is only apparent; it is also probable that it is especially prominent 

 in specimens kept in spirit, on account of the contraction. 



It has been correctly observed by Vosmaer in tPorifera (Bronn's Klassen und Ordnungen), 

 and by many other authors, that tlie terms pores and oscula have certainly been used of structures 

 far from always being homologous. Of late the term pore has often been used of the apertures of 

 the incurrent canals in the subdermal cavities, and then the pores of the surface have been called 

 dermal pores (Ostia Mi n chin). This I think unnecessary, as it does not come nearer the homology. 

 Vosmaer (Tijdschr. d. Nederl. Dierk. Vereenig. (2) III, 1890 — 92, 238) has proposed other terms, trying 

 to establish homologies with regard to the excurrent apertures, so that he calls the aperture with 

 which his vcloaca» opens outward, osculum, and the openings into this cloaca of the excurrent canals 

 procts, when there is only one opening, and proctions, when one canal has more than one opening; 

 in certain sponges the osculum is wanting, and then the apertures of the excurrent canals on the 

 surface are procts or proctions. In the same way he uses the term stomata for the pores on the sur- 

 face of the sponge, or stomions, if there are more than one pore for each canal. Hitherto he has 

 only tried to establish this terminology in the Hexactinellids and Tetractinellids; whether it may be 

 used there may be doubted, and at all events it cannot be used in the Monaxonids; in forms with 

 large subdermal cavities, for inst, it will be impossible to distinguish between stomata and stomions. 

 Therefore I always use the term pore of the incurrent openings on the surface of the sponge, while I 

 use the term osculum or oscula of the excurrent opening or openings on the surface, but in the latter 

 case it may sometimes be a matter of judgment what is to be called osculum. In a tuljular sponge 

 as a Siphouoclialiiia or many species of Reniei'a I call the opening of the tube osculum, and in a 

 leafshaped sponge with the excurrent openings on one side I use of those the term oscula. The diffi- 

 culty occurs at forms of transition between the above mentioned, as for inst. funnel-shaped or open- 

 calicular sponges, but in these cases the signification of the terms used will appear from the description. 

 Minchin is surely right, when he says (Lankester: A Treatise on Zool. II. The Porifera and Coelen- 

 terata, 1900 pp. 23 and 36), that most frequently these questions can neither be decided by outer study 

 nor by anatomical examination, but only by the knowledge of the development of the individual forms. 

 For the present we must therefore be satisfied with knowing that the terms used do not imply 

 homologies. 



Although, of course, the pores of a given sponge may be found in all degrees between open 



