the latter case generally perforate a thin pore-membrane stretched across 

 a larger canal or aperture. False pores may be similarly aggregated. 

 The whole pore-system is, properly speaking, inhalent, i.e. gives entrance 

 to currents of water, but when there is no osculum certain pores are large 

 and exhalent 1 . The osculum or proper exhalent vent is usually con- 

 sidered as the homologue of the mouth, a view beyond a doubt incorrect, 

 (pp. 801-2). It is at first single, but during growth the number undergoes 

 increase ; and then the new oscula may be either scattered or grouped 

 together. In leaf-shaped sponges they are more or less confined to one 

 surface, the pores to the other (Polejaeff ) a separation which is complete 

 in the Calcarean family Teichonidae. The aperture may be superficial, at 

 the apex of a papilla, depressed, or surrounded by a corona of spicules, 

 as in Homoderma and some other Calcarea. It is often limited by a con- 

 tractile membranous sphincter. Its size depends on that of the sponge 

 in great measure. In the Calcarean Asconidae it has approximately the 

 same diameter as the body, i. e. -I 2mm. : in Heterocoela it ranges from 

 i 2mm. or even more. The Non-Calcarea show great differences in this 

 respect, and oscula of 8 10 mm. (| in. circa) have been observed. The 

 absence of an osculum is known as lipostomy 2 . 



The gastric cavity conforms to one of four types, all of which occur 

 in Calcarea^ only the third and fourth in Non-Calcarea. (i) The cavity is 

 lined throughout by collared flagellate endoderm Calcarea Homocoela. 

 It is simple and more or less tubular in Asconidae and Leucopsidae ; in 

 Homoderma produced into a number of radial cones, thus forming a 

 transition to the next type. (2) The cavity is divided into a central 

 oscular tube lined by non-flagellate pavement cells, and a set of radial 

 cones to which the collared flagellate endoderm is confined. The cones 

 have a wide aperture into the oscular tube 3 ; the pores open on their 

 surfaces, and the spaces between them constitute an inhalent or * intercanal ' 

 system of channels. This type characterises the family Syconidae among 

 Calcarea Heterocoela. The cones may remain separate and independent, 

 Syconinae ; their walls may coalesce from place to place owing to the 

 growth of mesoglaea converting the inhalent canals into irregular branched 



1 See von Lendenfeld's figure of a section of Ascetta (Leucosolenia) Madeayi (op. cit. ante, ix. 

 PI. 62, Fig. 12, cf. p. 1147), where small pores open externally, and large into a pseudogaster : so 

 too in Leucopsis (ibid. p. 1089). He sometimes speaks of the inhalent canal closed by a pore-mem- 

 brane as the pore\ cf. Z. W. Z. xxxviii. p. 240-1. In Euplectella there are pores of large size 

 ( = oscula ?) in the body- wall which open direct into the central cavity of the sponge, and have no 

 connection to the gastric ampullae. 



3 The cause of lipostomy is generally assumed to be the obliteration of the osculum in 

 growth. It is possibly never formed in some instances. Euplectella is often quoted as an example, 

 but the sieve plate at the summit of the body contains ampullae identical with those of the 

 body-wall. 



3 Haeckel terms these apertures 'gastral ostia.' He supposed the cones to possess also distal 

 or ' dermal ostia/ non-existent according to more recent researches. 



