TIT 



PHYLUM AND CLASS PORIFERA 



117 



times the Sponge grows from the narrow base of attachment into 

 a thin flat plate or lamella ; this may become divided up into a 

 number of parts or lobes, which may exhibit a divergent arrange- 

 ment like the ribs of an open fan. Often the lamella becomes 

 folded, and sometimes there is a coalescence between the folds, 

 resulting in the development of a honey comb-like form of sponge. 



Sponges resemble plants, and differ 

 from the higher groups of animals, in 

 the readiness with which, in many 

 cases, their form becomes modified 

 during growth by external conditions 

 (environment). Different individuals of 

 the same kind of Sponge, while still 

 exhibiting the same essential structure 

 and the same general mode of growth, 

 may present a variety of minor differ- 

 ences of form, in accordance with 

 differences in the form of the support- 

 ing surface or in the action of waves 

 and currents. 



Leading Modifications of Struc- 

 ture. Sycon gelatinosum belongs to a 

 type of Sponges intermediate between 

 the very simplest forms on the one 

 hand and the more complex on the 

 other. The simplest type of Sponge- 

 structure is that of the so-called Ascetta 

 or Olynthus (Fig. 86). This is not a 

 mature form no adult Sponge retain- 

 ing such simplicity of structure. It is 

 vase-shaped, contracted at the base to 

 form a sort of stalk by the expanded 

 extremity of which it is attached ; at 

 the opposite or free end is the circular 



osculum. So far there is a consider- FlG . 86 ._ O iynttms stage of a simple 

 able resemblance to Sycon gelatino- calcareous sponge (ciathrina) A 



, ,, J , . ' ,, . portion of the wall of the vase-like 



Slim ; DUt the Structure 01 its Wall in sponge removed to show the para- 



Ascetta is extremely simple. Regularly 



arranged over the surface are a number of small rounded apertures, 

 the inhalant pores ; but, since the wall of the Sponge is very thin, 

 these apertures lead directly into the central or paragastric cavity 

 (Fig. 87, A), the long passages or canals through which the com- 

 munication is effected in Sycon being absent. The wall consists of 

 the same three layers as in Sycou, but the middle one, though it 

 contains a small number of spicules, is very thin. The ectoderm is 

 a thin layer of flat cells ; the paragastric cavity is lined throughout 

 by choanocytes similar to those of the flagellate canals of Sycon. 



