PORIFERA. 



81 



forms with no spongin and forms with no spicules in the spongin 

 fibres (Ceratosci). Which is the primitive condition if either is 

 it is impossible to say. The Ceratosa are an artificial order, its 

 families being related to different families of the Halicliomlrina. 

 It may, indeed, be looked upon as an assemblage of halichondrine 

 forms, in which the reduction of silica and development of spongin 

 have reached their extreme limits. The calcareous and silicious 

 spicules are secreted in the protoplasm of the mesodermal network ; 

 Scl&roblast is the term applied to the special uninucleated part of 

 the network in which they arise. They consist of an organic axis 

 which is generally continuous through the points of the spicules 

 with the adjacent organic structures, and of an organic sheath which 

 is presumably a remnant of the parent 

 scleroblast. The silicious spicules are 

 formed of opal (colloid silica). 



The spicules are of two kinds the 

 large spicules or megascleres (essential 

 spicules of Bowerbank, skeletal spic- 

 ules of Carter), and the small spicules 

 or microscleres (auxiliary spicules of 

 Bowerbank, flesh spicules of Carter). 

 The megascleres (Figs. 71, 72, and 

 78) are embedded in spongin fibres 

 which may be either reticulate or 

 radiating in arrangement ; or if there 

 is no spongin, they are held together 

 by strong connective tissue bands. 

 In addition there is usually a number 

 of megascleres scattered irregularly 



through the tissues. In the LitJiistida the spicules which are called 

 desmas are articulated together so as to form a network. The micro- 

 scleres (Fig. 73), which are not really sharply distinguishable from the 

 megascleres for the two pass into one another do not, as a rule, 

 take part in forming the supporting skeleton ; they are embedded in 

 the mesoderm and sometimes project into the canals. The spicules 

 near the surface of the body are often differently arranged to those 

 of the main skeleton. It is well known that some sponges shed 

 an immense number of spicules; and it appears that in many cases 

 the spicules are continually being moved towards the surface, where 

 they are cast off, and replaced by spicules formed in the central 

 parts of the sponge. 



G 



FIG. 70. Spongin-fibre of Euspongia 

 irregiilaris (from Perrier). K,/ axial 

 medulla of the fibre ; sp spongo- 

 blasts. 



