346 Dr. H. B. Roll— On Fossil Sponges. 



tubules, as for instance those in Jerea (Polypothecia) dicJiotoma 

 (Benett), and J. pyriformis (Lamour.), etc., do not differ from oscules 

 except in their greater length. 



4. The Fores. — Besides the oscules, palseontological writers are in 

 the habit of speaking of the pores. It must be understood, however, 

 that by this term they designate not the temporary openings in the 

 sarcode of the animal during imbibition, to which it is properly 

 applied, but merely the interstices in the tissue of the skeleton in 

 the dead sponge ; for in the living state these interstices are more or 

 less completely filled with the sarcode. In some sponges there are 

 no orifices either of incurrent or excurrent canals that are distin- 

 guishable ..either by form, size, or position from the ordinary inter- 

 stices of the sponge-tissue, the whole being formed of a nearly 

 uniform rete ; and in these cases the pores or interstices must sup- 

 ply the place of the larger orifices, although closed by the sarcode 

 during the intervals of active inhalation and exhalation ; moreover, 

 in those sponges possessing well-marked incurrent orifices, it is still 

 probable that the whole of the external surface is more or less an 

 inhalent one, through the interspaces of the rete, according to the 

 exigencies of the animal. 



5. The Epitheca. — Among the fossil sponges some portion of the 

 surface, especially externally towards the base, is frequently observed 

 to be either without pores, or they are so minute as to be invisible, 

 and the sponge then appears as though covered by a more or less 

 smooth or slightly wrinkled membrane, which has been regarded by 

 D'Orbigny, De Fromentelle, F. A. Eoemer and others, as analogous to 

 the epitheca of the Zoantharia ; and the occurrence of this epitheca 

 has been held to be an additional evidence of the stony nature of the 

 sponge skeleton. When examined microscopically, by means of thin 

 sections, however, it appears that this epitheca is due to the filling 

 up of the interstices of the superficial parts of the sponge, which, in 

 the situations in which it exists, is finer and more condensed than 

 elsewhere.^ This greater density at the surface may be seen in many 

 recent sponges, the superficial portions of the tissues being closer 

 and finer than that of the interior, which was formed during an 

 earlier and more active period in the growth of the sponge. But 

 either from having arrived at maturity, or at a period when the 

 growth was temporarily arrested, for in some sponges the growth is 

 intermittent, or, as appears sometimes to be the case, from some local 

 cause, the tissue at the surface assumes a closer arrangement. Thus 

 in the common Halichondria panicea, the surface over greater or 

 lesser portions frequently presents a condensed appearance with 

 scarcely any visible interspaces, the outer superficial portion being 



1 It is to this closer superficial portion of the tissue that M. Etallon has applied 

 the name of perienchyma. " Dans certains cas," he observes, " lorsque le spongiaire 

 parait avoir acquis tout son developpement, le tissue devient plus fin, plus serre, 

 recouvre toute la surface d'une couche plus ou moins epaisse et adhe'rente et donne 

 au squelette un aspect different de celui qu'il avait a I'e'poque de la croissance et 

 qu'on peut toujours retrouver par des coupes ouparl'usure."— i2ayowwe's<?M Oorallien 

 {Eaut Jura), p. 139; also Sur la Classification des Spotigiaires du Saut Jura, p. 137. 



