120 Mr. C. C. Nutting on the. 



idea of their homology. He thinks that tlie sarcostyles are 

 homologous with the capitate tentacles of many species of 

 hydroid polyps. 1 believe that he has no supporters in this 

 view. 



No other investigations of sufficient importance to discuss 

 in this connexion have been made, so far as I know, with the 

 exception of my own work in 1895 at Plymouth, England *, 

 and at Naples, where I made careful studies of these struc- 

 tures in the living plumularians and by means of serial 

 sections. 



The histology of the sarcostyles, as held by most of the 

 above writers, shows an outer layer of ectoderm and an inner 

 layer of endoderm, these two layers being separated by an 

 apparently structureless membrane called by the German 

 writers the " Stutzlamelle." The endodermal layer appears 

 to be a solid core in stained and sectioned preparations, and 

 is so described by most writers. Under favourable conditions 

 living specimens may be examined under a high power, and 

 by a proper management of light the cell-boundaries, muscle, 

 and indeed almost every histological detail may be distinctly 

 seen and the movements followed. It was while making 

 such examinations of living sarcostyles at Naples in 1895 

 that I found an unexpected proof that the axis of the sarco- 

 style is not a solid rod, but a delicate collapsible tube, the 

 cavity of which is strictly homologous with the body-cavity 

 of the hydranth. While studying a living sarcostyle under a 

 l^ oil-immersion lens, the endodermal axis, as it is called, 

 was very sharply defined, being separated from the ectodermal 

 layer by the " Stutzlamelle." Much to my surprise, I saw 

 an amoeboid cell pass quickly along the exact centre of the 

 axis. The cell was largely composed of highly refractive 

 granules and exhibited very active amoeboid movements, 

 sending forth well-marked pseudopodia and constantly 

 changing form. This mysterious cell appeared to be en- 

 gaged in travelling back and forth between the distal and 

 proximal end of the axial cavity of the sarcostyle. Its pro- 

 gress was unimpeded, and completely demonstrated to my 

 mind the presence of an axial cavity in the sarcostyle. After 

 having once seen this cell, I looked for them in other sarco- 

 styles, and found them in nearly every one examined. The 

 species under observation was Aglaophenia Helleri. I after- 

 ward found similar cells in the endoderm of various parts of 

 the plumularian colony, particularly in the stem. In such 



* See C. C. Nutting, " Notes on Plymouth Hydroids,'" Journ. Marine 

 Biol. Assoc, February 1896, p. 153. 



