348 ALICE W. WILCOX. 



brought out by Hyatt (65). The frequent division of the colo- 

 nies along with the tendency toward further growth occasions 

 locomotion: 



The real cause of locomotion must however be referred back 

 to the activity of the moving colonies and the condition of the 

 substratum on which they rest. The individual polypides of 

 Pectinate/la are very irritable. They contract and expand Ire- 

 quently and with some force. All the polypides on one side 

 of a colony contracting simultaneously give impetus enough to 

 move the whole colony over a slippery surface. The gelat- 

 inous secretion which underlies the young colonies has not yet 

 hardened, and in its semi-fluid state offers a slimy surface over 

 which the small colonies move with very little resistance. 



The rate and amount of locomotion varies with the size and 

 condition of the colony. After a colony is well started and 

 growing rapidly, it divides often and the resulting colonies move 

 apart quickly and as far as the free space about them permits. 

 But there are limits to this power of locomotion. The colonies 

 can move only as they are impelled by the activity of the poly- 

 pides. Hence as the polypides lose their vigor, as they do dur- 

 ing reproduction, locomotion must cease. The power of loco- 

 motion also decreases with the increasing size of the colonies. 

 The external conditions also limit locomotion. So long as the 

 gelatinous substratum is in a semi-liquid state, the young colonies 

 move with freedom, but as it gradually solidifies they become 

 fixed. Often locomotion is limited also by the space about the 

 colony. If two colonies in process of locomotion come in con- 

 tact with each other, their motion is checked and they fuse 

 together. As the colonies spread and form a confluent mass, 

 further growth is allowed for only by the rapid thickening of the 

 gelatinous ectocyst. 



But what is gained by this unique power of locomotion ? As 

 division is necessary to keep the individual colonies small, so 

 locomotion after division, is necessary to allow growth to pro- 

 ceed freely in all directions, and the characteristic radiately-lobed 

 colonies to be formed. 



This discovery of locomotion is significant, owing to the light 

 which it throws upon the mode of development of the large 



