148 SUMMARY. 



it may seem, it appears to me not impossible that the Cceleuterata may 

 have had an ancestor in which a digestive tract was physiologically replaced 

 by a solid mass of amoeboid cells. This ancestor was perhaps common 

 to the Turbellarians also. The constant presence of thread-cells in the 

 inner layer of their epiblast fits in with their derivation from a form 

 similar to the planula. While the solid pai'enchymatous digestive canal of 

 Convohita and Schizoprora and other forms amongst the Turbellarians, though 

 very probably secondary, may perhaps be explained by such a view of their 

 origin. 



The plauula in its primitive condition is not bilaterally symmetrical, 

 but frequently, as amongst the Actinozoa, it becomes flattened on two sides 

 before undergoing its conversion into the adult form. Perhaps the bilateral 

 form of planula is the starting point both for the Coelenterata and 

 the Turbellaria. In this connection the peculiar unilateral development of 

 a tentacle in Scyphistotna and Actinia should be noted. 



The planula occurs in the majority of sessile forms of Hydrozoa 

 except the Tubularidie and Hydra. It is also characteristic of the 

 Trachy medusae aucl Siphonophora. Amongst the Acraspeda it is 

 also present, but has an exceptional mode of ontogeny which is 

 discussed in connection with the germinal layers. 



It is characteristic both of the Octocoralla and Hexacoralla, 

 but is not found in the Ctenophora. 



In the Tubularidse and in Hydra an abbreviated development 

 leads no doubt to the absence of a free planula stage, and the 

 absence of a larval form amongst the Cteiiophora may, as has 

 already been stated, be probably explained in the same way. 



The Ccelenterata of all the Metazoa are characterized by the 

 greatest simplicity in the arrangement of their germinal layers ; 

 and for this reason very considerable interest attaches to the mode 

 of formation of the layers amongst them. Two germinal layers 

 are constantly found, which correspond in a general way to the 

 epiblast and hypoblast. It might have been anticipated that a 

 certain amount of uniformity would have existed in the mode of 

 formation of the layers. This however is not the case. In perhaps 

 the majority of forms they become differentiated by a process of 

 delamination, but in a not inconsiderable minority the two layers 

 owe their origin to an invagination. 



Delamination is constant (with the doubtful exception of some 

 Tubularidse) amongst the Hydromedusae and Siphonophora. It is 

 perhaps in the main characteristic of the Actinozoa. 



Invagination by einbole takes place, so far as is known, constantly 

 amongst the Acraspeda and frequently amongst the Actinozoa ; 

 and an epibolic invagination is characteristic of the Ctenophora. 



If confidence is to be placed in the recorded observations on 

 which this summary is founded, and there is no reason, why in a 

 general way it should not be so placed, the conclusion is inevitable 

 that of the above modes of development the one must be primitive 

 and the other a derivative from it, for, if this conclusion be not 



