30 EMBRYOLOGY 



moath of Coelenterates, farfchermore that the Porifera in 

 general are derived from a monaxial, heteropolar funda- 

 mental form in which the production of secondary axes in 

 definite numbers has not yet taken place, whereas the radial 

 type with four rays lies at the foundation of the Cnidaria 

 (compare F. E. Schdlze, No. 12 ; A. Goette, No. 6 ; Heider, 

 No. 8). The absence of movable processes of the body 

 (tentacles) and the low grade of histological differentiation 

 serve as substantiating facts in support of this view. 



The Porifera possess no true muscle fibres. The property of con- 

 tractility appears rather to belong to all the cells in about the same 

 degree, and the " contractile fibre-cells " occurring in the mesoderm of 

 many sponges are distinguished from true muscle fibres by the fact that 

 the contractile substance in them has not yet become separated as a 

 distinct portion of the cell. The absence of a nervous system has not 

 yet been proved, it is true ; but the presence of such a system does not 

 appear to be firmly established, for up to the present time the groups of 

 cells claimed by Lendenfeld as the nervous system of sponges have 

 remained doubtful as regards this interpretation. 



In respect to the origin of the canal system of sponges, 

 reference must be made to those primitive forms which are 

 found more especially among the calcareous sponges, and 

 by comparison of which it is most clearly proved that the 

 complicated canal system of the siliceous and horny sponges 

 has been evolved by a continuous process of folding of the 

 wall of the sacular olynthus-like primitive form, whereby 

 the collar epithelium eventually becomes localized in 

 particular parts (ampullae) of the canal system. If the 

 diagram Fig. 10 A represents the wall of a simple ascon 

 perforated by pores, and if we bear in mind that the entire 

 inner surface is covered with collar-cells, then Fig. 10 B 

 shows the origin of the radial tubes of a sycon by means 

 of a folding of this wall. At the same time the entoderm 

 lining the common central cavity is transformed into a pave- 

 ment epithelium ; and alternating with the evaginations of 

 the radial tubes, enfoldings (a) of the outer surface of the 

 body lined with ectoderm have also been formed, the funda- 

 ments of the inhalent canal system. Fig. 10 C shows how, 

 by a repeated process of folding, diverticular spaces (6) of 



