GENERAL REMARKS ON THE FORM AND STRUCTURE OF THE 



HEXACTINELLIDA. 



Although the different species of the Hexaetiuellida vary greatly in form and 

 structure, they nevertheless exhibit so essential an agreement in the fundamental 

 features of their organisation, that it is easy to reduce all known forms to a common 

 type. Apart altogether, for the present, from the skeletal portions, this common type 

 resembles a simple sac, in which the outer surface is formed of a thin skin rich in pores 

 (the dermal membrane), through which water enters into a space (the subdermal 

 trdbecidar space) crossed by fine trabeculse. Within this there is a single layer of 

 closely arranged chambers shaped like the finger of a glove, with wide internal openings, 

 and with fine walls (the memhrana reticidaris), supported by an elegant quadrate net- 

 work, and penetrated by small round pores {chamber pores) for the passage of water. A 

 connecting membrane extends between the internal openings of the chambers, and is in 

 direct continuation with the walls of the chambers. This membrane generally agrees in 

 sti'ucture with that bounding the chambers, and closes internally the clefts and apertures 

 which occur between the latter. Through the pores of the chamber walls and the 

 connecting membrane, the water reaches an inner or siibgastral trabecular space, 

 which is likewdse crossed by fine trabeculse, and is separated from the wide gastral cavity 

 by a porous or net-hke internal limiting skin, or gastral membrane. 



From the exterior inwards the following successive layers occur : — (1) the outer hmit- 

 ing skin or dermal membrane ; (2) the outer or subdermal trabecular framework ; (3) the 

 cAa?n6ers with their connecting membrane ; (4) the inner or subgastral framework ; (5) 

 the inner uniting or gastral membrane. 



The body of every Ilexactinellid is invariably made up of these five layers in the 

 same order, but the designation of the layers as outer and inner refers not so much to 

 their relation to the centre of the individual sponge, as to the direction of the stream of 

 water. Thus the dermal membrane always refers to that through which the water enters 

 the sponge body, and the gastral membrane, on the other hand, to that through wliidi it 

 escapes from the body-wall into the gastral space, or directly to the exterior. 



In the attempt to trace the manifold modifications of the numerous species of 



