382 EVENINGS AT THE MICROSCOPE 



but all together, and in the same direction, as if moved 

 by a single spring. A violent laceration of the polypary 

 causes these polypes to remain extended and stretched like 

 a waving and tremulous fringe across the mouth of the 

 shell, for several minutes. The ophidian polypes (evi- 

 dently a barren modification of the reproductive polype) 

 are never found in any other situation on the polypary 

 than in that before described, or round the margins of ac- 

 cidental holes in the shell. They have no mouth, and the 

 tentacles are rudimentary. The walls of the body are very- 

 transparent, from the extreme vacuolation of the inner tis- 

 sue. The muscular coat, as might be expected from the 

 active movements of the polypes, is highly developed, and 

 forms a beautiful object on the dark polarized field of the 

 microscope, each spiral coil shining out as a bright double 

 ring, divided by four dark sectors. The outer tissue of 

 the whole body and tentacles is crowded with the larger 

 thread-cells. The ophidian polypes are, doubtless, organs 

 of defence or offence, like the motile spines and bird's- 

 head processes of the Polyzoa, or the pedicellarise of the 

 Echinodermata; but it is difficult to assign a reason for 

 their peculiar situation. They vary much in number and 

 size in different specimens of Hydr actinia, but are rarely 

 altogether absent." ' 



The reflections of the able zoologist who first called 

 attention to these varied developments, and his compari- 

 sons of them with those of another polype- form which we 

 have lately been observing, are so interesting and instruc- 

 tive that you will not deem it needful that I should apolo- 

 gize for citing them. "In our consideration of the Hy- 

 dractinia," he observes, "our attention is arrested by the 



1 Dr. Wright, op. cit. 



