MATERIALS FOR A MONOGRAPH OF THE ASCONS. 529 



1. Observations upon Siliceous Sponges. — For the 

 spicules of Monaxonida the classic researches of Lieberkiihn 

 and Carter established the fact so far back as the fifties that 

 the spicules of Spongilla originated each within a single 

 cell. At a still earlier date Carter had been of opinion that 

 spicules and horny skeleton were '^ formed in the intercellular 

 substance^ and not within the cells ;^^^ but direct observations 

 caused him to abandon this opinion. Lieberkiihn gave a full 

 and clear description of the formation of the spicules of 

 Spongilla in 1856 (pp. 407—409, figs. 17—30) ; and a year 

 later, but independently. Carter published an account con- 

 firming that given by Lieberkiihn (1857, p. 23, fig. 8). Carter 

 afterwards extended his observations to marine sponges, and 

 described a similar origin for the spicules in Esperia sega- 

 gropila, Jnstn. (1874 [1], pp. 101—105, and [2] p. 456, 

 pi. xxi, fig. 26), and in Microciona armata, Bwk. 

 (1874 [2], p. 457, pi. xxi, fig. 27). Schmidt had already 

 (1864, p. 5, Taf. i, fig. 13) described the formation of the 

 spicules of Reniera, sp., and he is quoted by Sollas as 

 describing in detail their origin and growth in Esperia, in a 

 work which I have not been able to see.^ Kolliker also figured 

 and described the monaxon spicule in its formative cell (1864, 

 p. 61, woodcut 15), and Keller described the spicule formation 

 in the embryo of Chalinula fertilis (1879, p. 334, pi. xix, 

 fig. 20). Sollas gave further instances of spicule development 

 in a series of papers dealing with sponges of Norway ; the 

 bundles of trichites originating, each bundle, in one cell (1880, 

 pp. 141, 142, pi. vii, fig. 21) ; the formation of the sterras- 

 ters of Geodia Barretti, Bwk., each in a single cell (1880, 

 pp. 256 and 401, pi. xi, fig. 18, and pi. xvii, figs. 18 — 22) ; 

 and the monaxon spicules arising each in a cell (1882, p. 159, 

 pi. vii, figs. 12, 17, 18). It is not necessary to multiply these 

 instances further ; all observers are of one accord in describing 

 the monaxon spicules and microscleres of Demospongise gene- 



> 'Ann. and Mag. Nat. Hist.' (2), iv, 1849, p. 95. 



' ' Zoologische Ergebnisse der Noidenfahrt,' p. 120 ; see Sollas (1888), 

 p. xlv. 



