313 ANATOMY AND PHYSIOLOGY 



skeleton is a Ml and complete but elongate aggregation of 

 particles of sand, each separately coated by keratode, 

 forming a series of stout anastomosing fibres, consisting of 

 innumerable extraneous molecules encased by a tbin coat 

 of keratode. 



In Dysidea KirMi, an Australian species, both the pri- 

 mary and secondary fibres of the skeleton are comparatively 

 large, frequently exceeding half a line in diameter. In our 

 British species, Dysided fragiUs^ Johnston, the primary 

 fibres are often as abundantly arenated as those of the 

 Australian species, while the secondary ones are only par- 

 tially filled with extraneous matter, and in this condition 

 they are frequently more or less ttibidar. The structure 

 and pecuHarities of the above-named two species are de- 

 scribed in detail in vol. i, p. 63, plate vi, of the ' Transac- 

 tions of the Microscopical Society of London.' Fig. 270, 

 Plate XIV, represents a portion of one of the skeleton fibres 

 of Dysidea fragilis, Johnston, exceedingly full of sand, 

 X 108 linear. Fig. 273 exhibits the mode in which a 

 fibre takes up and envelopes a particle of sand, X 108 

 linear ; and Fig. 271 represents a small piece of the sponge 

 in its natural state, X 108 hnear. 



ON THE DISCRIMINATION OF THE SPECIES OF THE 

 SPONGIAD^, 



% 



One of the reasons why so little progress has been made 

 in our knowledge of the Spongiadse, is that the generic and 

 specific characters that are visible to the unassisted eye, 

 such as form and cblour, are in this class of animals 

 remarkably uncertain and delusive, while all those that are 

 definite and constant require not only a high degree of 

 microscopical power to make them visible, but frequently 

 also a peculiar mode of treatment to render them apparent 

 even beneath the microscope. Thus it is with many of the 

 finer forms of stellate spicula, which are very characteristic 

 in Tethea, Geodia, Spongilla, and other genera. When we 

 search for them by the dissolution of the tissues in nitric 



