OBSEKVATIONS ON POLYZOA. 197 



and the activity of the fissiparous function of the ccenoe- 

 cium. Even upon a level surface the continued multipli- 

 cation of the crencecia, in course of time, crowds them 

 together and presses the central colonies gradually up- 

 ward, forcing them to excrete more gelatine, and thus 

 the central portion of the general ectocyst becomes the 

 thickest. 



Thus the thickness of the ectocyst is not a regularly 

 progressive character, and the fluctuation appears to be 

 caused by the peculiarity of the base of the attachment, 

 and the multiplication of ccenrecia by self-division in 

 Pectinatella. 



Parchment-like or gelatinous. This is the condition 

 of the ectocyst in all the Fredericelke, even the color- 

 less varieties are tougher than the colorless varieties of 

 Plumatella. The Alcyonnelloid varieties of Plumatella 

 are more generally surrounded by a gelatinous ectocyst 

 than the diffuse varieties. 



Brown, or colorless. These characters accompany the 

 preceding, the parchment-like being always brown, and 

 the gelatinous always transparent. 



Permanent throughout life. The parchment-like ecto- 

 cysts of Fredericella long survive the decay of the poly- 

 pides, but the gelatinous disappear soon after their de- 

 cease. The same rule obtains- until we reach Pectinatella. 

 In this genus, the thickness of the mass causes it to com- 

 mence decaying before the colonies die, and they often 

 are found loosely scattered on the surface after it has de- 

 cayed to a thin sheet. This gives to an old settlement of 

 the Pectinatelte a Cristatellian-like aspect, since in the 

 latter the ectocyst is thin. 



The old colonies of Pectinatella, however, do not lose 

 the power of excreting gelatine, but readily refasten 

 themselves after being detached. The loss of permanency 

 in the ectocyst of Pectinatella is fittingly followed by its 

 appearance in Cristatella as a transient excretion. 



Envelopes the entire coenoecium. This character has 

 been treated of in mentioning the cause of the natural 

 position of the coencecium. It is strictly decrements!, 



