QUEKETT MICROSCOPICAL CLUB. 91 



plication arises chiefly from budding and colony formation. 

 The degree of complication of the canal system may be taken 

 as an indication of the degree of evolution of the whole sponge. 

 An ordinary bath sponge is a more complicated organism and 

 it is impossible to say how many individuals it represents. 

 You cannot define individuality. Sponges may be divided into 

 five great groups : first, those with calcareous spicules, the 

 Calcarea ; second, those which have no skeleton at all, the 

 Myxospongida, very primitive forms ; third, the Triaxonida, 

 with skeleton composed of siliceous spicules having three 

 axes and six rays ; fourth, the Tetraxonida, with siliceous 

 spicules having four axes and four rays, radiating at equal 

 angles from the centre ; fifth, the Euceratosa, in which the 

 skeleton is composed of horny fibres and there are no spicules. 

 The spicules originate in mother cells. The mother cells 

 are found in the mesogloea, the substance between the ectoderm 

 and the endoderm. They are not always the product of one 

 cell only. There is a spicule which reaches the length of 3J 

 feet, and is of nearly the thickness of a lead pencil. One 

 mother cell could hardly give origin to this, and it is not known 

 how it arises. Siliceous spicules are formed of opal, secreted 

 by the protoplasm of the cell. They are arranged in a scattered 

 manner in the simplest cases. By reference to diagrams, it was 

 shown how from the primitive tetraxonid form with four rays 

 — like a calthrops — a great number of shapes are derived. One 

 ray may be elongated while the others remain short (triaenes) ; 

 three rays may be suppressed, leaving only one, which may 

 acquire a head. Many other varieties were described and illus- 

 trated by sketches and by large diagrams. In most sponges 

 the spicules are divided into two categories, skeleton spicules 

 (megascleres) and flesh spicules (microscleres). Flesh spicules 

 are of interest because it is so difficult to conjecture what can 

 be their use. They are very beautiful and constant in form. 

 A great number of shapes were pointed out on the diagrams, 

 the method of their origin from simpler forms being described 

 and illustrated on the blackboard. Prof. Dendy had brought 

 for distribution among the members a tube containing a sponge 

 from the Indian Ocean — Dercito'psis minor — ^which would afford 

 interesting examples of some of the more primitive tetraxonid 

 spicules. 



