REPORT ON THE HEXACTINELLIDA. 47 



NOMENCLATURE AND TECHNICAL EXPRESSIONS. 



Dermal membrane. — The external limiting membrane, through the pores of which the 



water enters the body. 

 Dermal pores. — The large and small pores which perforate the dermal membrane. 

 Suhdermal traheculas. — The delicate strands of tissue which form an irregular framework 



extending between the dermal membrane and the chamber layer. 

 Subdermal trabecular space. — The space between the dermal membrane and the 



chamber layer, and partly traversed by the subdermal trabecular framework. 

 Chamber layer.— The more or less folded layer of adjacent ciliated chambers. 

 Connecting membrane. — The continuation of the chamber wall, stretched between the 



terminal openings of the chambers. 

 Chamber pores. — The small round apertures in the chamber wall. 

 Gastral membrane. — ^The internal limiting membrane directly surrounding the gastral 



space. 

 Gastral pores. — The pores of very varied size which perforate the gastral membrane. 

 Subgastral trabeculse. — The delicate strands of tissue which are united into an irregular 



framework, extending between the chamber layer and the gastral membrane, and 



also frequently into the efferent canals. 

 Subgastral trabecular space. — The space between the chamber layer and the gastral 



membrane, which is partly traversed by the subgastral trabecular framework. 

 Terminal sieve-plate. — A sieve-like perforated plate, which extends over the broad 



terminal opening of many tubular or cup-shaped Hexactinellida. 

 Parietal gaps. — Apertures over the whole external waU of the Sponge, through which 



the space within communicates directly with the surrounding medium. 

 Membrane of the parietal gaps. — An iris-like, circular membrane stretched across the 



gaps, with circular bands of muscular fibres, by means of which the orifice may be 



narrowed or entirely shut. 

 Covering plate. — A porous plate which surrounds like a capsule the body of some forms 



with tubular framework, and which is united only to the terminal oscular opening 



of the tube. 

 Spiculum. — Every independent and originally isolated skeletal element. 

 Principal ray. — The primary ray which springs directly from the central nodal point of 



a spicule. 

 Terminal ray. — The branch or secondary ray springing from the outer end of a 



principal. 

 Hexact, pentad, tetrad, triad, diad, monact. — Nouns and adjectives used to designate 



the spicules according to the number of their principal rays. 



