xlviii THE VOYAGE OF H.M.S. CHALLENGER. 



empirical sense to designate the openings on the surface of the sponge through which the 

 water is discharged from the canal system. In this sense the word is now generally 

 understood."' 



Thus in the case of tuljular sponges, like Siphonochalma (PI. VII.), the osculum 

 is the large opening at the top of each tube. In the case of cup-shaped sponges, however, 

 such as Tedania infundihuliformis (PI. XI. fig. 1), we cannot call the wide expanded 

 mouth of the funnel an osculum, although probably it is homologous with the opening 

 of the tube in Siphonochaliyia. In flabellate sponges, as we have already had occasion to 

 point out (p. xxxix), the oscula are usually confined to one surface; these flabellate forms 

 by folding round give rise to funnel-shaped sponges — e.g., PhakeUia ventilahrum and 

 Phyllospongia (which may be either flabellate or cup-shaped), — in which the oscula 

 are usually situated on the inner surface of the funnel. By a further extension of the 

 same process the sponge becomes tubular, with a comparatively narrow opening at the 

 top which w^e now call the osculum. How far this folding and complication may be 

 carried we do not know. 



The oscula in the Monaxonida vary very greatly in size ;" they may either be small and 

 abundant {e.g., Petrosia hispida, PL III. fig. 2 ; PhakeUia ventilahrum, var. connexiva, 

 PI. XLIX. fig. 3 ; Esperiopsis challengeri, PI. XVIII. fig. 1 ; Homceodictya grandis, PI. 

 XXII. figs. I, \b, &c.), or large and comparatively few {e.g., Esperella murrayi, PI. XIV. 

 figs. 1, la, Spirastrella papillosa, PI. XLI. fig. 5, &c.). They are very commonly 

 found on the uppermost part of the sponge, as in Halichondria latruncidioides 

 (PL I. fig. 5), JEsp>ereUa murrayi (PL XIV. figs. 1, la), Sp)trastrella solida (PL XLI. 

 fig. 7), &c., but their distribution is very variable. Their form also varies much ; they 

 may either have their margins level with the general surface of the sponge and not 

 surrounded by any raised collar, as, for example, in Pacliychalina {?) punctata (PL VI. 

 fig. 2), or they may be surrounded each by a membranous collar, as in Esperella 

 lapidiformis (PL XVI. figs. 2, 2b), or they may be raised on the summits of papiUse, as in 

 Latrunculia apicalis (PL XLIV. fig. 4 ; PL LI. fig. 1). They may be merely the 

 openings of shallow, basin-like depressions, into which numerous exhalent canals discharge 

 Ijy numerous small mouths, as in Poxhyclialina fibrosa (PL IV. fig. 3); or they may be 

 the openings of long, wide canals coming up from deep in the body of the sponge, as 

 in Latruncidia apicalis (PL LI. fig. 1). Sometimes they are arranged in stellate 

 groups, a condition which appears to be most common in flabellate sponges {e.g., 

 Homceodictya grandis, PL XXII. ; and PhakeUia flabellata, PL XXXIV. fig. 3a); or, 

 again, there may be only a single osculum to each individual, as in Stylocordyla 

 (PL XLIII. fig. 10). 



' Cf. Vosmaer, Bronn's Klass. u. Ordnung. d. Thierreichs, Porifera, p. 127. 

 2 For measurements see the Description of Genera and Species. 



