GENERAL ZOOLOGY 



sponge of the Atlantic coast, can be squeezed through silk bolting cloth of 

 very fine mesh, so that the cells of the sponge are almost completely separated 

 or dissociated. If these dissociated cells are allowed to settle upon the 

 bottom of a dish of sea water and remain undisturbed, they immediately begin 

 to coalesce into small multicellular masses. These masses continue to fuse, 

 forming thin encrustations over the substratum, and under favorable condi- 

 tions large sponges like the original will eventually be formed. The 

 phenomenon of reassociation involves very interesting problems. It 

 demonstrates with remarkable clarity the relatively loose organization char- 

 acteristic of the sponge body; what is particularly remarkable is the fact 

 that the capacity of reforming their original organization resides in isolated 

 cells. A similar ability to reassociate occurs to a limited extent in some 

 simple coelenterates, and such phenomena can be demonstrated in embryonic 

 stages of more highly specialized animals (p. 167). 



The loose association of cells in the sponge body, together with the rela- 

 tively slight degree of specialization exhibited by the somatic cells of sponges, 

 is a cogent reason for considering sponges to be at the cellular level of organi- 

 zation, in contrast with the unicellular protozoans and the tissue-grade organ- 

 isms of more advanced phyla. 



The Sponge Body Plan 



To speak of the sponge body raises the question of what constitutes the in- 

 dividual in animals of this phylum. In the olynthus, or in such a sponge as 

 Scypha, an individual with a single osculum is recognizable. In other 

 sponges, which grow into large masses with many oscula, the entire mass may 

 be called an individual; or, the individual may be defined as any part of the 

 entire mass that includes an osculum and its related canal system. However 

 we define the individual sponge, the body plan basic for the phylum is the 

 one seen in the olynthus stage and in its simpler derivatives. In the cellular 

 organization of the sponge body, there are tissues of a simple kind but no 

 organs. The early stages of development are peculiar, and it is difficult to 

 reconcile the layers of the sponge body with the embryonic germ layers of 

 more complex animals. Finally, the sponge has no internal cavity homologous 

 with the digestive cavity or enteron of higher animals. This last feature 

 of bodily structure, together with the primitive cellular organization and the 

 peculiar mode of development, gives the phylum its unique position among 

 the Metazoa. 



Evolutionary Significance of Mesozoa and Porifera 



The fact that both Mesozoa and Porifera are multicellular organisms and 

 show .some cellular difTerentiation warrants placing them among the Metazoa. 



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