SKELETON OF C<ELENTERATA. 



105 



Skeleton. 

 § 81. 



In addition to the organs of support, which are formed by the 

 above-mentioned tests, the Ccelenterata have various other forms of 

 skeletons, which are differentiations of the mesoderm. 



There are none in the Physemariaa, which make up for the 

 absence by taking up foreign bodies into their ectoderm j in the 

 Porifera, some of which have no firm structures (Halisarcina), organs 



Fig. 36. A portion of the surface of the body of a Calcareous Sponge (Sycaltis 

 perforata) to show the triradiate spicules, o Dermal ostia, each of them surrounded by 



a circlet of spicules (after Hackel). 



of support are formed by firm needles (spicula), or softer fibres, 

 which are placed in the mesoderm. The former are formed of chalk, 

 or of silica ; the sponges are known accordingly as Calcareous or 

 Siliceous sponges. The spicules of the Calcispongiee are simpler in 

 character, for they are either acicular tri- or quadri-radiate ; they 

 present great regularity of distribution and arrangement, together 

 with numerous modifications in individual characters. The above 

 figure gives a representation of the spicula of a Calcareous Sponge. 

 The hard structures, when formed of silica, are much more varied in 

 form. In addition to the acicular structures, which are combined 

 together in various ways to form multiradiate stars, there are 

 various other kinds of firm parts, as, for example, the amphidiscs 



