OBSERVATIONS ON POLYZOA. 211 



parent in the young, and acquire the pergameneous char- 

 acter only by age. I do not here allude to the color 

 acquired simply by the gathering on the surface of minute 

 particles of sediment, but the deep brown color due not 

 only to these particles, but to the horny condition of the 

 ectocyst produced by age. The increase in the gelatinous 

 character of the ectocyst in Plumatella may be safely said 

 to be in part due to the shorter lives of those varieties, 

 and species that are habitually transparent ; but the change 

 in Cristatella cannot be accounted for in any such way. 



In order to see how this happens, it is necessary to 

 notice another series of characteristics. These are the 

 horizontally of the coenoecium in Pectinatella, due to the 

 withdrawal of the support of the ectocyst, and its con- 

 centration below the coenoecium, which last is probably 

 caused by the same changes. in the outer membrane, that 

 enlarge the size of the cells. The increase in the size of 

 the minute cells, composing the tissue, are the only os- 

 tensible changes noticeable in the membrane secreting the 

 ectocyst, to which their sudden cessation of the excretory 

 powers on the upper side might be referred. 



In Cristatella the ectocyst still continues to be ex- 

 creted only from the lower side, and the introduction of 

 its power of locomotion enabling it to secrete a new ecto- 

 cyst as it goes, always maintains the latter new and soft. 

 Pectinatella has no locomotive muscles, and, therefore, 

 the ectocyst accumulating under ;t is tougher than in 

 Cristatella, though it remains gelatinous in consequence 

 of its great thickness. 



The modifications of the variable character of the 

 lophoric region, as has been said, are dependent upon the 

 increment in the length of the arms. 



The variable characteristics of the entire group of 

 Phylactolsemata, as may be conceived from the preceding, 

 are dependent for their modifications or changes upon 

 (1) the increase in the number of buds developing from 

 the original cell, (2) the increase in the number of indi- 

 viduals having the gelatinous ectocyst, (3) the changes or 

 agencies that remove it from the upper side of the coenoe- 

 cium, (4) the bending inwards of the third and fourth 



