Hyatt.] 158 [March 5, 



gastrula. During this evolution the mesenchyme became more 

 and more important, and as a result of its thickening the habit of 

 budding was more or less suppressed so that the higher types must 

 be considered as individuals with highly plastic forms, liable to 

 excessive outgrowths, but not as branching colonies. The arch- 

 enteron also remains unchanged throughout life, or gives rise to 

 simple diverticula, or in forms with thicker mesenchyme the diverti- 

 cula themselves form branching tubes. The fact that no coelom or 

 body cavity is formed, in spite of the opportunity offered by the 

 increasing thickness of the mesenchyme is very significant. It is 

 not yet established that the mesenchyme does receive some addi- 

 tions in course of its growth from the endoderm and ectoderm, but 

 so far as the histology is now understood it is doubtful. In other 

 words the Porifera are intermediate with regard to structural com- 

 position between primitive larval individuals, like the free larvae 

 of all colonial types, and the differentiated colonies which arise 

 from such primitive individuals after they become attached, as in 

 the Hydrozoa. They contain all the elements necessary for- the 

 formation of complicated colonies with complete zooids of all kinds, 

 sexual and asexual, in all their varieties as displayed in Hydrozoa ; 

 but in consequence of the less differentiation of the mesenchyme 

 their primitive individuality is maintained and the processes of 

 budding take place internally and externally without perfect cor- 

 relation. That is, the exterior has outgrowths and so has the 

 archenteron, but these are not strictly coincident and produce true 

 buds only in the forms with thin mesenchyme. 



The evidence in favor of the opinion that the diverticula or am- 

 pullae are strictly homologous with the archenteric diverticula of 

 all other animals is very strong. The young have no diverticula un- 

 til the ampullinula is formed and this correlates with the absence 

 of these organs in the adults of the lowest type, Ascones. These 

 facts among sponges seem to be in accord with the history and 

 development of the diverticula among Hydrozoa and Actinozoa. 



This has led to the conclusion that in all three of these types 

 the diverticula are homoplastic organs. 



The considerations we have presented above have, therefore, a 

 direct application to the results of the work done of late years by 

 Semper, Dohrn and others in tracing the origin of the vertebrata 

 to some worm-like type. The whole of this evidence hangs neces- 



