532 B. A. MINCHIN. 



which the shaft of a large spicule has a series of pyriform 

 cells along it^ considered by the author to be derived from the 

 '^collenchyma" (pi. iv, fig. 30; pi. v, fig. 6a; pi. viii, fig. 37; 

 compare p. 12). Sollas further states that in the Lithistida 

 the crepis of the desma is formed in a single scleroblast, but 

 that other cells are found on the arms of the desma. " Each of 

 the four depressions which occur about the centrum in the 

 angle between the arms of a tetracladine desma appears to be 

 occupied by a scleroblast, and others may possibly be distri- 

 buted along the sides of the arms " (p. xlvi). Reference is given 

 to figures (pi. XXX, figs. 20 and 21) showing the minute tetrac- 

 tines, each enclosed in a cell. Finally, Leudenfeld (1894 [1], 

 p. 166) expresses an opinion with regard to the tissues and 

 asters of Geodia and Ancorina, which is the same as that 

 which Sollas enunciated in more general terms. He believes 

 the spicules to arise within single cells as a refractile body near 

 the nucleus, afterwards embracing it. Some of the cells have 

 a process and are believed to form trioenes; others show a 

 radial striation and form asters. No figures are given in 

 support of these statements. 



From a study of the literature of siliceous spicules, and 

 without going beyond the statements of the authors, one 

 would be justified, I think, in making the following generalisa- 

 tions : — (1) Every spicule originates within a single cell, 

 which perhaps in many cases receives assistance afterwards 

 from other cells, and certainly does so in the case of the 

 complicated desmas of the Lithistida. No evidence for 

 division taking place in the mother-cell or in its nucleus. 

 (2) The forms with few rays are derived from those with 

 many, by reduction and loss of some of the rays. This 

 applies in the first instance to the spicules of the Hexacti- 

 nellids and Tetractinellids, but it perhaps holds good also 

 for the phylogeny of the simple spicules of the Mo- 

 naxonida. 



(2) Observations on Calcareous Sponges. — The earliest 

 recorded observations upon the relations of calcareous spicules 

 to the tissues of the sponge are those of KoUiker (1864) upon 



