EEPOET ON THE MONAXONIDA. xlvii 



We have never succeeded iu detecting the inhalent openings of the chambers, nor, 

 so far as we are aware, has any one else done so in a Monaxonid sponge. Possibly this 

 is due partly to the fact that they are excessively minute and close up almost or quite 

 entirely when the sponge is treated with reagents ; and also to the difficulty in obtaining 

 very thin sections of siliceous sponges. That they exist there can be no reasonable 

 doubt, and there is no need for us to take refuge in the novel theory, recently pro- 

 pounded by Mr. Carter,^ that the flagellated chambers (" ampuUaceous sacs") have each 

 only a single opening. 



(5) The Exhalent Canal System. 



We have been obliged, in speaking of the flagellated chambers, somewhat to forestall 

 our observations on the exhalent canal system, and we have little to add. It is usually, 

 at any rate in its finer ramifications, lacunar like the inhalent system ; so that we have 

 a system of interdigitating, ramifying lacuna, some inhalent and some exhalent, 

 separated from one another by the mesodermal tissues in which the flagellated chambers are 

 embedded. This appears to be always the condition in the Halichondrina (PI. XLVIII. 

 fig. 2cZ ; and PL XLIX. fig. 2), and, sometimes at any rate, in the Clavulina (PI. LI. 

 fig. la). But, as already pointed out, we may also, in the Clavulina, find narrow exhalent 

 canaliculi leading away from the flagellated chambers, as iu Stylocordyla (PL L. fig. la) 

 and Polyinastia ( Weberella) bursa. The small exhalent channels gradually unite together 

 into wider and wider and usually more and more definite canals, which ultimately open 

 on to the surface at the oscula. Even in species in which the ultimate ramifications of 

 the exhalent system are lacunar, the larger canals are usually perfectly distinct and 

 definite, as, for example, in Esperella murrayi (PL XLVIII. fig. 2, e.c.) and Latrunculia 

 apicalis (PL LI. fig. 1) ; and they are occasionally (e.gr., Spirastrella solida) provided with 

 very definite, circular diaphragms, occurring at intervals, for the purpose of regulating 

 the outflow of the water; while at other times {e.g., Latrunculia apicalis and Stylo- 

 cordyla stipitata, var. glohosa) they are surrounded by a sheath of fibrous tissue which 

 probably serves the same purpose. 



(6) The Oscula. 



The structures described under this name by various authors are not, in many cases, 

 as pointed out abeady by Vosmaer, homologous one with another. Indeed, in the 

 present state of our knowledge it is impossible to unravel the intricate question of the 

 homologies of the oscula. In this work we shall, therefore, use the term in a purely 



' Ann. and Mag. Nat. Hist, ser. 5, vol. six. p. 203, et seq. 



