HTDEOIDS FKOM WOOd's HOLL, MASS. 359 



The polyps of the European and American Gemmaria, how- 

 ever, exhibit differences enough to remain generically separate ; 

 such are the form of the polyp, the degree of dififerentiation into 

 hydranth and hydrocaulus, the relative development of perisarc, 

 and the arrangement of tentacles. Here, again, as has been 

 pointed out by others, we are confronted by the anomalous con- 

 dition of two medusse, almost identical, being produced from 

 polyps generically separated. 



Marine Biological Laboratoky, 



Wood's Holl, Mass.; August, 1898. 



EXPLANATION OF PLATE 34, 



Illustrating Mr. L. Murbach^s paper on " Hydroids from 

 Wood's Holl, Mass.'' 



Hypolytus. 



Figures 1—10 are eight times enlarged. 



Fig. 1. — Adult male polyp, a. t. Aboral tentacles, c. Nettle collar. ^". 

 An immature goiiophore. g' . Gonophore with sperm nearly ripe. he. Hydro- 

 caulus. h. Hypostome. hy. Hydranlh. o. t. Oral tentacles, p. Processes 

 at the bases of the gonophores. 



Fig. 1 a — g'" . A female gonophore. o. Ova. p. The process at the base. 



Fig. 2. — Foot end of a polyp showing constrictions (a, b) preceding fission. 



Fig. 3. — The same with one blastolyte {b) free, and the other (a) showing 

 a lateral thickening. 



Fig. 4. — The same with both blastolytes free. 



Fig. 5. — The same showing the hydranth of the young polyp {b) formed 

 from the side of the adult. The forked foot was formed by approximation of 

 tapering ends of the blastolyte («). 



Fig. 6. — Young polyp normally formed from the blastolyte {b). 



Fig. 7. — Foot end of another polyp («) showing lateral thickening on 

 blastolyte (6). 



