PORIFERA. 805 



lobularis. The papillae of the surface, or the whole sponge, if small, may 

 be converted into globular vesicles or brood-buds, 2-3 mm. in diameter. 

 The wall of such a bud is composed of ectoderm, mesoglaea and endoderm. 

 It contains semi-globular ampullae, which open externally by narrow pore- 

 canals, internally by a single aperture. Its internal cavity is the dilated 

 exhalent canal system of the papilla or sponge, as the case may be. These 

 brood-buds float about, but finally settle down, flatten out, and give rise 

 each to a new sponge. 



With the exception of Spongillidae, the Porifera are marine. They are 

 found in all seas. The Calcarea are cosmopolitan. The majority of 

 Hyalospongiae live at depths greater than 150 fathoms, often descending 

 below 400, as do the Lithistina below 100. Most Halichondrina and 

 Ceratina are restricted by a zone of 50 fathoms, the Calcarea of 100, but 

 Ascetta (Leucosolmia] blanca has been dredged at 450. Sponges frequently 

 shelter animals of other groups, especially Crustacea, within their canal 

 systems. As to size, they vary extremely, some attaining a maximum limit, 

 others differing much. The Calcarea are small ; e. g. the mean size of the 

 Asconidae is 13 mm., of the Syconidae and Leuconidae 1520 mm. (i.e. 



in.). The goblet sponges, Poterion, are the largest known. P. Amphi- 



d 



tritae may attain a height of 3^ ft., and its cup a diameter of 2j ft. The 

 Calcarea are colourless ; other sponges are variously tinted, but usually of 

 one colour. The Spongillidae possess chlorophyll ; see pp. 242-4, 251. 

 The yellow pigment Aplysinofulvin turns blue, then black on exposure to 

 air. In some instances coloration is due solely to algae living in the 

 mesoglaea 1 . Many sponges exhale a strong and peculiar odour somewhat 

 resembling oxydising phosphorus or ozone 2 . Fossil forms are numerous. 

 As to Calcarea, a Syconidan, Protosycon, has been found in Jurassic strata, 

 and an extinct group, the Pharetrones of Zittel, represented by a single 

 species in the Devonian, is greatly developed in the Mesozoic period, and 

 dies out in the Eocene 3 . Of the Non-Calcarea, the Hyalospongiae and 

 Lithistina appear iiv the Silurian, and attain their maximum development 



1 See the table in Vosmaer, Porifera, p. 458 ; Brandt, Mittheil. Zool. Stat. Naples, iv. p. 223, 

 and Carter, A. N. H. (5), ii. 1878. According to Brandt, Zooxanthella (see p. 243, ante) is found in 

 Hircinia variabilis and Reniera cratera. 



2 See Krukenberg, Vergleich. Physiol. Studien (i), 2, p. 37, and cf. p. 44. The smell is pro- 

 bably due to an ethereal oil. 



3 Inasmuch as lime often replaces silica and vice versd in the fossil spicules of sponges, the 

 position of the Pharetrones has been much debated. But the complete accordance of their spicules 

 with those of living Calcarea, the fact that they may be oval or rhomboidal in section, whilst true 

 siliceous spicules are round, has led the most recent authorities to support Zittel's view. See von 

 Dunikowski, Palaeontographica, xxix. 1882-3; Hinde, A. N. H. (5), x. 1882, and Catalogue of 

 Fossil Sponges, Brit. Mus. 1883, p. 157 ; Sollas, Sci. Proc. Royal Dublin Soc. iv. 1885, p. 387, and 

 cf. pp. 389-90. Calcarean spicules occur in Pleiocene beds (Hinde, Quart. Journ. Geol. Soc. xlii. 

 1886, p. 214). For ILne replacing silica, see Sollas, A. N. H. (5), vi. p. 437 et seqq., and vice, 

 versd, Id. Quart. Journ. Geol. Soc. xxxiii. pp. 252-4, 813-19, 835 ; cf. Hinde, Ph. Tr. 176, pp. 425-33. 



