338 EVENINGS AT THE MICROSCOPE. 



wards, and then quickly roll themselves up again ; and 

 that not irregularly or independently of each other, 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 (evidently 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 accidental 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 tissue. The 

 muscular coat, as might be expected from the active move- 

 ments of the polypes, is highly developed, and forms a 

 beautiful object on the dark polarised field of the micro- 

 scope, 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 pedicellarias of the Echi- 

 nodermata; but it is difficult to assign a reason for their 

 peculiar situation. They vary much in number and size 

 in different specimens of Hydractinia, but are rarely alto- 

 gether 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 

 apologise for citing them. "In our consideration of the 

 Hydractinia," he observes, "our attention is arrested by 

 the multitude of objects grouped together to constitute a 

 single animal, their variety in form, and the sympathy 

 * Dr. Wright, op. cit. 



