ZOOPHYTES. 385 



Tlie expliintitiou 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 in- 

 habit 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 then develop all the 

 polypes and organs proper to the complete organism. 

 When this is the case, an admirable opportunity is 

 presented, for studying with ease and precision the 

 economy of the creature ; and it is to tlie skill with 

 which Dr. T. Strethill Wright has availed himself of 

 such an opportunity* 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 

 transparent 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 

 curiouslv rido;ed with toothed keels ; and these ridges 

 run in various directions over the horny layer also, 

 makino- a fine network over it. The investing 

 fiesh, however, fills up all the cavities and areas so 

 inclosed. 



The mode in which the polypary increases is by 

 tlu-owing out from its edge a creeping band, exactly 

 analogous to the root-thread of the Laomedea. This 

 " propagative stolon, after leaving the point of its 

 origin, increases rapidly in diameter, and sends out 



* See Edin. New Phl'osophical Journal, for April, 1857. 

 17 



