^52 



together, assisted hy the dotvngrmvth of the body -ivall form the 

 stomodoeuni. The latter increases in depth with the size of 

 the polyp, and the process probably goes on to some extent 

 throughout life. 



It is necessary to lay strong emphasis on the fact that tJiis 

 is a larval developnicnt* The organs are functional, and the 

 conditions of life are not quite the same as in the adult. All 

 the organs are correlated with one another ; the endoderm is 

 digestive and everywhere ingests foreign particles. t In this 

 stage there is a necessity for a rapid growth, for an abundant 

 nutrition, which might he assisted by the widely open mouih. 

 Perhaps there is no need of protection, so that the tentacles 

 only appear later on to guard the polyp when it is producing 

 generative organs. On the other hand the endodermal 

 nematocysts are possibly now or were in the evolution of 

 these forms at one time functional. 



If a large number of the raadreporaria be examined, I 

 believe that in a not inconsiderable number a corresponding 

 development will be found. The condition was probably 

 brought about by an enormous enlargement of the gastro- 

 pore in the first place, the stomodoeum then becoming of 

 secondary formation. In the adult the stomodoeum is pro- 

 bably a definite morphological entity. Primitively it arises 

 as an in pushing or ingrowth of the ectoderm at a very early 

 stage of development. The adult stage is necessary to the 

 organism. The variation has been produced and perpetuated 

 in the young stage alone. This larva cannot be held to 

 indicate in any way an ancestral stage of structure that has 

 been hit upon by natural selection. It shows rather a 

 variation on an entirely new line. The case is one which 

 markedly emphasises the fact that the tendency in develop- 

 ment is " to directness and abbreviation and to the omission 

 of ancestral stages of structure," which here is as true for 

 larval as for embryonic development. 



* Vide " On the Law of Developuient commonly known as Von Baer's Law, and 

 on the Significance of Ancestral Rudiments in Embryonic Development " by 

 Ad^m Sedgwick, Qwirt. Jour. Micro. Sfi., vol. 36, and Studies Mo>ph. Lah. Camh., 

 vol. vi., pp. 75-92 ()S9G). 



t Perhaps the n matocysts in the endoderm at the edges of the teotacles were 

 once functional at this stajje in protecting the tissues of the polyp. 



