Classification of the Spongida. 187 



become so branched and altered as to be undistinguisliable but 

 for the presence of the trifid central canal. 3. Acerate spicule 

 of the body, beam-like, scattered among the foregoing, long, 

 smooth, curved, finely pointed or fusiform, and acuate. Flesh- 

 spicules of various forms, sometimes two or more in the same 

 species, viz. acerate smooth or microspined, cylindrical bent 

 and more or less inflated towards the ends, microspined, or 

 sinuous and blunt-spined tubercle-like. 



Family o. Potamospongida. 

 Group 19. Spongillina. 



Sarcode colourless, greenish or purple. Skeleton composed 

 of a reticulation of spiculo-fibre of two kinds, viz. vertical or 

 large, and horizontal or small. Structure radiating, more or 

 less plumose ; interstices filled up by areolar sarcode, flaky 

 when dry, charged with the spicules of the species and the 

 ampullaceous sacs. Colour pale tawny yellow, greenish, or 

 purple. Vents large, scattered irregularly. Branched excre 

 tory canal-system well pronounced. Pores spread generally over 

 the surface, in the sarcode which tympanizes the interstices of 

 the dermal reticulation. Texture friable, crushable, crumb-of- 

 bread-like. Spicules of two kinds, viz. skeleton- and flesh- 

 spicules. Skeleton-spicule acerate, curved, smooth, more or 

 less finely pointed. Flesh-spicule of various forms, according 

 to the species. Characterized by the presence of reproductive 

 seed-like bodies, visible to the naked eye, and of a globular 

 or ellipsoidal form (according to the species), with a hiliform 

 depression opening into the interior, corresponding in colour 

 with that of the species, only brighter or more intense ; com- 

 posed of a horny capsule surrounded by a columnar structure 

 of horny cells, or by a layer of flesh-spicules arranged perpen- 

 dicularly or tangentially (according with the species) to the 

 surface ; filled with germinating substance of the sponge, 

 which, under growth, makes its exit through the hiliform 

 opening. Habitat. Fresh water. 



HEXACTINELLIDA. 



For the characters of the Groups into which the Families of 

 the Hexactinellida have been divided, see the illustrations 

 respectively pp. 199 and 200 ; and for all known species, see 

 1 Annals,' 1873, vol. xii. p. 349 &c. 



