344 A. Hyatt — Larval Theory of the Origi7i of Tissue. 



tion and importance of this layer in the morphology of the 

 gronp. 



We have also tried to show that the general morphology and 

 development indicated the gradual evolution of series of forms 

 from a type similar to Ascones but without a skeleton, which 

 we have considered directly comparable, as stated by Heeckel, 

 with the gastrula. During this evolution the mesenchyme be- 

 came more and more important, and as a result of its thicken- 

 ing the habit of budding was more or less suppressed so that 

 the higher types must be considered as individuals with a 

 highly plastic form, liable to excessive outgrowths, but not as 

 branching Metazoons. The archenteron also remains persistent 

 throughout life, gives rise to simple diverticula, or, in forms with 

 thick mesenchyme, diverticula themselves form branching tubes. 



The fact that no internal column 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 additions 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 composition between primitive larval individuals, 

 like the free larvse 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, 

 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 

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

 the archenteron, but these are not strictly coincident and pro- 

 duce true buds only in forms with thin mesenchyme. 



The evidence in favor of the opinion that the diverticula or 

 Ampullae are strictly homologous with the archenteric diver- 

 ticula of all other animals is very strong. The young have no 

 diverticula until 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 ac- 

 cord with the history and development of the diverticula 

 among Hydrozoa and Actinozoa, and leads to the conclusion 

 that in all of these three types the diverticula are homoplastic 

 organs, and not found in the lowest forms of these groups or 

 in the early stages of development of the normal forms. 



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 

 vertebra ta to some worm-like type. The whole of this evi- 



