150 NOTES ON PLYMOUTH IIYDROIDS. 



discussion, particularly by Hincks, Allmau, Reichart, Merejkowsky, 

 Weismann, and Jickeli. With the excellent facilities afforded by the 

 Plymouth laboratory, and the valuable suggestions of its director, I 

 secured a number of fortunate serial sections of the expanded sarcostyles 

 of P. pinnata, and have been able to satisfy myself concerning the main 

 points of their structure. The results of this study have corroborated 

 the statements of Merejkowsky up to a certain point, including the 

 following facts — 



1. The greater part of the sarcostyle is composed of ectodermal cells. 



2. There is a central endoderm core (or cylinder ?) * 



3. The cells on one side of this core are very large and quadrangular, 

 while the cells on the other side and of the entire terminal portion of 

 the sarcostyle are of ordinary size, 



4. There are pseudopodia-like processes from the free surface of the 

 sarcostyle. 



On the other hand, I have been entirely unable to find any trace of 

 the "interstitial protoplasm" described by Merejkowsky, in which he 

 claims that the ectodermal cells are imbedded. Weismann f boldly 

 suggests that this "interstitial protoplasm" is owing to an assumed 

 necessity for free sarcode to explain the pseudopodia-like processes on 

 the free surfaces of the sarcostyles. It seems to me that there is no 

 logical demand for free protoplasm to explain the great extensibility 

 of these organs. The possible tenuity of the walls of ectoderm cells 

 can be appreciated by any one who has made a study of nematocysts, 

 and a careful examination of the sarcostyles, both living and in serial 

 sections, has failed to afford any evidence of free protoplasm, and this 

 negative result is not antagonized by any physical necessity for free 

 protoplasm in organisms which can construct endoderm cell-walls of 

 the marvellous tenuity and extensibility of the nematocysts. 



The function of the nematophores is in more doubt than their 

 structure, and is not yet understood. It is practically certain that they 

 are more or less degraded " persons " of the colony which have come to 

 subserve definite functions of great service, judging from the prevalence 

 of these structures throughout the Flunudaridcc. So far as the species 

 under consideration is concerned, it is safe to say that the sarcostyles 

 are not " fighting persons " or " machopolyps," because they are not 

 armed with any considerable number of nematocysts, the special 

 weapons of hydroids. An examination of the living and active sarco- 

 styles establishes the following facts — 



1. The almost incredible extensibility of these organs, which wind 



* Whilii at Naples, the writer was able to demonstrate that this structure, in another 

 species, has a central cavity. 



t I^ie Entstehung der Sexualzelhn bei den Hydromcdusen, p. 176. 



