In leaf-shaped sponges they are more or less confined to one surface, the pores to the other (Polejaefif) - 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 contractile 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 1 - 2mm. or even more. The Non-Calcarea show great differences in this respect, and oscula of 8 - 10 mm. (2/5 in. circa) have been observed. The absence of an osculum is known as lipostomy2.

1 Judging from von Lendenfeld's figures, the ectoderm is also ciliated in Homoderma, Leu-candra maeandrina (Calcarea); in Bajalus, Aulena villosa and two species oiEuspongia-, see Proc. Lin. Soc. New South Wales, ix. PI. 65, Fig. 33; PL 67, Fig. 43; ibid. x. PI. 2, Fig. 4; PL 34; PL 37; PL 38, Fig. 2. The ectoderm cells are said to be sometimes not traceable, a fact due, perhaps, to faulty modes of preparation; see Harmer, On silver staining of marine objects, Mitth. Zool. Stat. Naples, v. 1884, p. 445. A cuticula has been described in some sponges, e. g. by Schulze, in some specimens of Euspongia officinalis (Z. W. Z. xxxii. p. 626), and on spots of the surface in Aplysina aerophobia where the ectoderm had been accidentally removed (Z. W. Z. xxx. p. 392). Von Lendenfeld traced the formation of a similar structure in Aplysilla violacea when the ectoderm was destroyed, derived from the superficial gland cells of the mesoglaea (Z. W. Z. xxxviii. pp. 255-6). Ianthella (Darwinellidae) has a distinct cuticle according to Polejaeff, Keratosa, Challenger Reports, xi. p. 373 It is figured by von Lendenfeld as flagellate in Leucandra maeandrina, op. cit. supra, ix.

PL 67, Fig. 43. Haeckel states (Kalkschwamme, i. p. 144) that the endoderm is multilaminar in certain Asconidae. No such arrangement has been found in any sponge examined by modern methods. An amoeboid condition of the endoderm cells has been observed in some Calcarea; cf. Metschnikoff, Z. \V. Z. xxxii. p. 362.

The gastric cavity conforms to one of four types, all of which occur in Calcarea, only the third and fourth in Non-Calcarea. (1) 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 tube3; 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 channels, and the surface of the sponge is smooth, Uteinae; and with the same coalescence the cones themselves may become branched, Grantinae1. Two sets of pores are to be distinguished in consequence of the establishment of the branched inhalent set of canals: the pores at the surface of the sponge, and the pores by which the canals open into the cones.

A transition to the next type is indicated in the Uteine Amphoriscus elongatus, where a certain number of cones may have a common aperture into the oscular tube. (3) The collared flagellate endoderm is confined to sac-like chambers or ampullae of large size, which have a single wide aperture of about the same diameter as the ampullae themselves, into an exhalent canal. The ampullae are also set radially or vertically to the cavity of the canal. This type is seen in the Sylleibid genus Polejna (Calcarea Heterocoela)2; in Euplectella; in Halisarca Dujardini, where the ampullae are often branched, and Bajalus; in Spongelia, Velinea, Aplysilla, and to a less marked, degree in Dendrillaz. The differences depend on the degree of complication observable in the inhalent and exhalent canal-systems which are present universally in this and the following type. The exhalent canals are short wide tubes, Polejna (Leucilla) connexiva; short tubes somewhat branched, P. (L.) titer; long tubes, simple in Bajalus, branched in Halisarca Dijardini, in Aplysilla, Dendrilla, Velinea, and Spongelia. The inhalent canals are more or less branched, wide in Halisarca, with pore membranes and subdermal cavities (post) in Bajalus and the Ceratine genera (? Velinea). Both systems of spaces are remarkably large, and the mesoglaea is reduced to mere anastomosing strands in Euplectella. The ampullae communicate with the inhalent canals as a rule by many pores, scattered over their surface, Polejna, Euplectella, Spongelia, or aggregated at the end opposed to the exhalent aperture, e. g.

Bajalus, Aplysilla. The Plakinidae are transitional to the next type, the ampullae frequently communicating with the exhalent canals by a short tube (Vosmaer). In other respects they offer various degrees of complication, e. g. in Plakina monolopha the ampullae may open outwards by a superficial pore, whilst Plakortis simplex has subdermal cavities. (4) The ampullae are small, either pyriform with a funnel-shaped exhalent pore-canal, or spherical with a tubular canal. When the inhalent canals are wide, the ampullae have many inhalent pores; when fine and tubular, several tubes may open into the same ampulla. This fourth type is found among Calcarea Heterocoela in the Leuconidae, and so far as is known also in the Teichonidae and in all thoroughly investigated Non-Calcarea except those under (3) supra1. The inhalent and exhalent canal systems are branched and of considerable extent. The superficial pores may lead either directly into ampullae or into canals which branch only slightly, e. g. Oscarella lobularis; into canals which first unite and then branch, e.g. Chondrosidae; through pore-membranes either directly or indirectly by a narrow canal into a more or less extensive system of subdermal spaces, from which branching canals take origin - a very general mode.