184 THE MOSS-ANIMALS, 
Side by side with these, occurred thin patches of gela- 
tine covered with what at first appeared a different spe- 
cies of Pectinatella. The central spaces of the colonies, 
however, were long and narrow, and much less brilliant, 
being surrounded by tawny-colored fringes of Polyzoa. 
This genus discards even the remnant of a branch which 
we mentioned in the lobes of the Pectinatella, and is a 
hollow sac flattened into a disc below, by which the whole 
colony move upon the gelatine or ectocyst as one animal. 
In Fredericella, the hard, parchment-like condition of 
the ectocyst was owing wholly to the age of the colony ; 
in the young, it was gelatinous. 
We have seen, also, that Lophopus was buried in 
its own ectocyst, which remained gelatinous throughout 
life, and that- the Pectinatelle, though firmly attached, 
simply rested on theirs. And we now see Cristatella 
making the last step in this process, becoming entirely 
independent of its ectocyst, which is only a transient se- 
cretion thrown off from the creeping disc, like slime from 
the foot of a snail, to smooth the path over which it 
crawls. In large settlements the colonies lie closely to- 
gether, but it is not infrequent to meet with a stray one 
wandering by itself. Locomotion is accomplished by a 
complete net-work of muscles within the sac. These, with 
perhaps other muscles in the walls, enable them to ex- 
pand the dise in any direction, and then secreting gela- 
tine, and holding to what they have thus gained, draw up 
their remaining portions. They move so slowly, how- 
ever, that minute colonies require a day to get over an 
inch on the side of a smooth glass dish, the larger colo- 
nies: progressing even more sluggishly. In Plate 5, the 
outline of a single polypide is given, with a portion of the 
net-work of internal muscles. : 
