B. Porifera incalcaria. II; Spiculispongiae. 5 



(about 50 pgg.) is paid to the skeleton. The mineral matter of (siliceous) spi- 

 cules is colloidal silica, consisting of 6. 1-7.34 /o water and 93.9-92.66% silica. 

 The spicnle consists of a central organic axis, surrounded by concentric layers 

 of opal, the outermost of which is invested in a spicule sheath of organic matter. 

 The whole nomenclature of the spicules is altered [in accordance with Ridley & 

 Dendy, and v. Lendenfeld; cf. Bericht f. 1887 Porifera p 2, 3]. The mega- 

 scleres are divided into 5 groups : monaxons, tetraxons, triaxons, polyaxons and 

 spheres. Strongyloclad and tyloclad appear to be identical with Ridley & Den- 

 dy's cladostrongyle and cladotylote. These are diactine monaxons rounded or 

 knobbed at one end , terminating at the other in two or more secondary actines 

 or cladi. The oxyclad differs in being pointed at the unbranched end. To one 



of these three groups belong the triaenes, in which the number of cladi 



is limited to three, which make with each other an angle of I20. Varieties of 

 the triaene are: anatriaene [= M. ta. <p(90], protriaene [= M. ta. <>)90] , 

 orthotriaene [=M.ta.(f> = 90], dichotriaene [= M.ta.d.bif.}. The amphitriaene 

 is branched at each end, the centrotriaene in the middle. The characteristic mega- 

 scleres of the Lithistida are distinguished as desmas. It is supposed that the 

 axial rod of an ordinary spicule early suffers an arrest of development, and that 

 on this fundamental spicule (crepis) at first concentric layers of silica are depo- 

 sited, which, however, subsequently grow out into irregular branches and tubercles. 

 The crepis may be either a monaxon or a tetraxon. In the former case the spi- 

 cule is called a rhabdocrepid desma ; in the latter case a tetracrepid desma. 

 Others, which cannot be derived from these, are acrepid. Real tetraxons are 

 the calthrops and the triod, which correspond to Haeckel's tetra- and trisceles. 

 The microscleres, from which the megascleres have been derived, are to be di- 

 vided into 2 series, viz. spires and asters. The spires have the form of a 

 screw helix ; to these belong: -- sigmaspire (= </>), toxaspire (= ^4), poly- 

 spire (= spirilla Crtr.), sigma (= c/>), toxa (=-^), chela, globule. Generally 

 the spicules develope in special cells (scleroblasts), one in each; but several 

 may be secreted together in the same scleroblast, and are then known as dragmas. 

 The asters are divided into two subsections: euasters and streptasters, ac- 

 cording as the radii or actines proceed from a centre or from an axis. In each 

 group 5 forms are distinguished. Chapters on the phylogeny of the scleres 

 (= spicules) and on their arrangement and distribution are given, but of these it 

 is almost impossible to make a short abstract. Classification. S. holds to 

 his opinion that the Porifera belong to a special sub-kingdom (Parazoa). The 

 groups Calcaria and Incalcaria are retained, but the names replaced by Mega- 

 mastictora and Micromastictora. The latter class is divided into 3 subclasses : 

 1) Myxospongiae ( Micromastictora which are askeletosew), 2) Hexactinellida 

 M. in which triaxon spicules contribute to the formation of the skeleton), and 

 3) Demospongiae (M. which possess a skeleton either of siliceous spicules or 

 spongin, or of both combined, but the megascleres are never triaxons ). The 

 Demospongiae contain Monaxonida [see infra] and Tetractinellida. The latter are 

 characterised by the fact that some or all of the scleres are tetraxons, triaenes 

 or desmas. There are two orders: Choristida and Lithistida. These primary di- 

 visions are all made with regard to the megascleres and their arrangement. For 

 the subdivision great value is attached to the microscleres. Thus we have 3 sub- 

 orders of the Choristida: 1) Sigmatophora (the microsclere, when present, is a 

 sigmaspire), 2) Asterophora (the microsclere is an aster), and 3) Microsclerophora 

 only microscleres, no megascleres). The Sigmatophora have two families : 

 Tetillidae and Samidae. The former is distinguished by the characteristic well- 

 known protriaenes ; a cortex is sometimes present, sometimes the ectosome is a 



