792 THE ANIMAL KINGDOM. 



except Immediately round the nucleus. Each cell bears a flagellum in 

 Oscarella (Halisarca] tabular is, Plakina monolopha, Aplysilla violacea, and 

 Dendrilla 1 ', and in Halisarca Dujardini the cells appear to change into 

 a slimy layer. The pavement epithelium lining the inhalent cavities and 

 canals (pp. 7935) is probably always, in some cases certainly, of ecto- 

 dermic origin. The endoderm cells are typically flagellate, the base of the 

 flagellum surrounded by a clear collar an extension of the exoplasm of 

 the cell. The endoplasm is variably granular and pigmented : it generally 

 contains vacuoles (? contractile). The cells are large in the Calcarea 

 (012 mm.), small in Non-Calcarea. Their shape is not the same in all 

 sponges : they are as a rule closely apposed, but in Etiplectella are planted 

 apart in diagonal lines, connected, however, by slender cords of protoplasm. 

 This typical endoderm is restricted, except in Calcarea Homocoela, to limited 

 portions of the gastric cavity, and is reduced in the oscular tube and 

 exhalent cavities and canals (post) to a pavement epithelium, flagellate 

 in Oscarella lobularis, Plakina monolopha, and Aplysilla violacea (at least 

 in part), but usually non-flagellate 2 . Columnar cells are sometimes found 

 where the two epithelia pass into one another. The mesoglaea is scanty 

 in Calcarea, especially in Homocoela, and most particularly so in the 

 Hyalospongian Euplectella. It is soft or firm, hyaline or granular ; it 

 imbeds the skeleton, and contains cells of many kinds, some of which are 

 contractile or muscular, whilst others have lately been regarded as sensory 

 and nervous (pp. 806-7). 



The gastral system is one of the most characteristic features of the Pori- 

 fera ; it opens externally by pores and oscula. The pores are very numerous, 

 round or oval in shape ; minute as a rule, e. g. in Asconidae, -01 02 mm. 

 as a mean ; sometimes large and small (macro-, micro-pores) in the same 

 sponge ; and in many instances capable of being closed either partially 

 or completely. They are either scattered or aggregated in areae, and in 



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 of Euspongia ; see Proc. 

 Lin. Soc. New South Wales, ix. PI. 65, Fig. 33 ; PI. 67, Fig. 43 ; ibid. x. PL 2, Fig. 4; PI. 34; 

 PI. 37; PI. 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), 

 lanthella {Darwinellidae} has a distinct cuticle according to Polejaeff, Keratosa, Challenger Reports, 

 xi. p. 37. 



2 It is figured by von Lendenfeld as flagellate in Leucandra maeandrina, op. cit. supra, ix. 

 PI. 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. W. Z. xxxii. p. 362. 



