ZOOPHYTES. 335 



it; and though their tentacles wave and stream hither and 

 thither, they are not retracted on this account. But just 

 touch with the point of the pencil in your hand any part 

 of the shaggy fleece, and instantly the whole colony retire 

 together, as if by a common impulse, apparently shrinking 

 into the substance of the shell. Yet they soon re-appear, 

 one after another quickly protruding its closed tentacles, 

 which are presently expanded as before. 



The explanation of this phenomenon is, that the whole 

 colony of polypes are but the free points, or feeding mouths, 

 of a common living film, which invests the shell; just as, 

 in Laomedea, the polypes that inhabit the vase-like cells 

 are the off-shoots or free points of the common medulla. 



The investing film will sometimes in captivity spread 

 upon the glass side of a tank, and there develop all the 

 polypes and organs proper to the complete organism. 

 When this is the case, an admirable opportunity is pre- 

 sented for studying with ease and precision the economy 

 of the creature; and it is to the skill with which Dr. T. 

 Strethill Wright has availed himself of such an oppor- 

 tunity* that I am indebted for the chief part of the facts 

 that I am going to tell you, connected with the form and 

 appearance, of which you can here judge for yourself. 



The spreading film or polypary is a thin coat of trans- 

 parent jelly, slightly coloured with various tints, which 

 secretes and deposits within its substance a still thinner 

 horny layer of chitine. This rises here and there into 

 numerous spines and points, which are curiously ridged 

 with toothed keels; and these ridges run in various 

 directions over the horny layer also, making a fine net- 

 work over it. The investing flesh, however, fills up all 

 the cavities and areas so inclosed. 



The mode in which the polypary increases is by throw- 

 ing out from its edge a creeping band, exactly analogous 

 to the root-thread of the Laomedea. This "propagative 

 * See "Edin. New Philosophical Journal," for April, 1857. 



