224 THE AMERICAN. NATURALIST.  [Vow. XXXII. 
The hydroids have been carefully studied by so few zoologists 
that it may not be amiss to define the sarcostyle more ex- 
plicitly before discussing it. A careful investigation of any 
plumularian hydroid will disclose the fact that beside the 
hydrothece containing the hydroid polyps or hydranths, there 
are numerous usually minute chitinous cups containing an 
exceedingly interesting structure, which in life is characterized 
by amazing extensibility. Allman in 1864 described it as 
“a soft granular mass which could send forth very extensible 
processes capable of being greatly protruded, and then so com- 
pletely retracted as to apparently disappear. These processes 
have the power of sending forth pseudopodia, as does the 
amoeba, and act in many respects as do certain -rhizopods.”’ 
This author considered that these processes were composed of 
protoplasm, pure and simple. 
When, however, the more refined and modern histological 
technique was applied by Hamann to the investigation of the 
sarcostyles, it was discovered that they were much more com- 
plicated structures than was at first supposed; that they were 
made up of several histological elements ; namely, an ectodermal 
layer surrounding an axial portion composed of endodermal 
cells, the ectoderm and endoderm being separated by a struc- 
tureless membrane or “ Stutzlamelle.”” The distal part of the 
sarcostyle contains nematocysts or nettling cells. This author 
(Hamann)! concluded that the enormous extensibility of the 
sarcostyle was due to muscle fibrillæ, and regards the entire 
sarcostyle as a degraded person or hydroid polyp in which the 
mouth and body cavity have been obliterated. 
In the same year, 1882, C. de Merejkowsky? announced that 
the histological elements were ectoderm and endoderm with a 
dividing membrane, and that the motile part was composed of 
ectoderm alone, the ectodermal cells being immersed in a 
contractile structureless protoplasm. To this latter substance 
he attributed the enormous extensibility of the sarcostyle 
and the pseudopodia-like processes originally described by 
1 Der Organismus der Hydroidpolypen. Jenaische Zeitsch. f- Naturw., Bd. xV, 
pp- 17, 18, 65. 
2 Arch. de Zool. Exp. et Gen., vol. x, pp. 583-610. 
