224 THE AMERICAN MUSEUM JOURNAL 



choanocytes and their digestible parts absorbed. What is left 

 is discarded and flows with the current out through the osculum 

 at the summit of the vase. 



The mesoderm (mes.) is a thin jelly-like layer between the 

 ectoderm and endoderm. It contains scattered amoeboid cells 

 and the reproductive elements, and is the origin of the skeleton. 



2. The Canal Systems. 



In the form of sponge just described the mesoderm is 

 extremely thin, but if, as in the majority of sponges, there is a 

 greater or less thickening of this layer, the pores will no longer 

 be perforations, but will become transformed into tubes or canals 



[fig. 9. — A SYCON SFONQE (.Graiitia ciliaia Fleming) GROWING ON A MUSSEL SHELL 

 Star-shaped colonies of the Ascidian Rotryllus are also growing on the same shell. 



(See p. 22 1, Fig. 3), which may branch and be modified in 

 various ways. This gives rise to three general types of sponges 

 which are therefore based mainly on the arrangement and varia- 

 tions of the pore- and canal-systems. These are known as 



(a) The Ascon Type, 



(b) The Sycon Type, 

 c) The Rhagon Type. 



' [10] 



