O ARTHUR DENDY. 



This peculiar external form is to some extent paralleled by 

 certain species of the genus Phyllospongia amongst the 

 horny sponges, as will be evident on referring to Lendenfeld's 

 figures (6). By far the most remarkable parallelism, however, 

 IS exhibited by a Renieriae species, not yet determined, which 

 so closely resembles a young Grantia labyrinthica in 

 external appearance that I at first placed it along with other 

 specimens of that sponge to be figured, and only found out 

 my mistake on microscopical examination. This interesting 

 example of homoplasy serves well to show the necessity of 

 microscopical examination before even the approximate posi- 

 tion of any particular sponge can be safely determined. 



Both surfaces of the cup are smooth, and the inner surface 

 is at once seen to be perforated by innumerable minute and 

 closely placed apertures, the exhalant openings of the flagel- 

 lated chambers. These give to the surface a minutely punc- 

 tate appearance, which is absent immediately below the free 

 margin, where the sponge wall becomes very thin and translu- 

 cent. On the outer surface of the cup the pore-sieves form 

 less obvious markings. 



(c) The skeleton. 



The Spicules. — The calcareous spicules composing the 

 skeleton of the sponge are of three main types — triradiate, 

 quadriradiate, and uniaxial (oxeote). Each of these types 

 occurs in the sponge under more than one modification, 

 according to its position. 



Triradiate Spicules. 

 These form the main mass of the skeleton. The different 

 modifications which they present in different parts of the 

 sponge depend chiefly upon the relative length of the rays ; 

 and, to some extent, upon the proportions of the angles 

 between them. Thus we find a more or less gradual series 

 between approximately equiradiate and equiangular spicules 

 on the one ^hand, in which all three rays are of about the 

 same length, and the jangles between them nearly equal (fig. 



