xvni STRUCTURE OF CRISTATELLA 495 



The tentacles are about eighty to ninety in number, and they 

 are, as in other Phylactolaemata, united at their bases by a 

 delicate web. The lophophore is horse-shoe-shaped (Fig. 236, 3) 

 throughout the group, with the exception of Fredericella, in which 

 genus it is circular. 



In some Phylactolaemata the polypide has been observed to 

 interlace its tentacles, so that the plume becomes a kind of cage, 



FIG. 217. Cristatella mucedo Cuv. (a small colony), R. Yare, above Norwich, x 24. 



in which the more active Infusoria are imprisoned until their 

 struggles have so far weakened them that they are swept into 

 the mouth by the action of the cilia of the tentacles. 1 



Around the edge of the Cristatella is found a zone of budding 

 tissue, which gives rise continuously to new individuals. Now, 

 whereas in Gymnolaemata the growing edge gives rise to zooecia, 

 whose cavities become completely cut off from that of the older 

 ones ; in Phylactolaemata the partitions between the zooecia are 

 never completed. The body-cavity of Cristatella is thus a con- 

 tinuous space, interrupted at the margin only by vertical septa 

 (see Fig. 247), which represent the partitions between the 

 zooecia of other forms. 



The body-wall consists of two epithelial layers of ectoderm 



and mesoderm, between which is a layer of muscular fibrea 



1 Hyatt, Proc. Essex Institute (U.S. A.) (reprint from vols. iv., v. 1866-1868), ].. 9. 



