496 POLYZOA 



Parts of the epithelium lining the body-cavity are ciliated. 

 Into the common body-cavity hang the polypide-bnds at the 

 edge of the colony, and the mature polypides in the more central 

 regions. There are usually three rows of polypides on either 

 side of the middle line, in the neighbourhood of which is an 

 area devoid of polypides, but containing " brown bodies " and 

 statoblasts. The polypides nearest to the middle line pass in 

 succession into the condition of " brown bodies," while young 

 buds near the margin grow up coincidently to form new 

 polypides. 



The movement of the colony is in the direction of the long 

 axis, although either end may go first. Sir John Dalyell records 

 an observation x on a specimen (about one inch long) which was 

 artificially divided into two halves. The two halves " receded 

 from each other as if by common consent," and were nearly an 

 inch apart in twenty hours. 



An observation made at Cambridge on a small colony of 

 about 7 mm. in greatest length gave the following results. 

 The colony moved 13 mm. (nearly twice its own length) in 

 8|- hours: in the next 40 hours it moved 20 mm. (^ inch); 

 while in the following 24 hours it moved only 6 mm. Large 

 colonies change their place only with reluctance. 



The locomotive power possessed by Cristatella is not unique 

 among Phylactolaemata. Lophopus, the first fresh-water Polyzoon 

 of which any description was published, was originally described 

 by Trembley in 1744 under the name of the " Polype a pannache." 

 Trembley observed the spontaneous division of the colony, 

 follotoed by the gradual separation from one another of the 

 daughter-colonies! 2 The power of dividing spontaneously is also 

 possessed by colonies of Cristatella and of Pectinatella. 



The colonies of Lophopus are surrounded by an excessively 

 hyaline ectocyst, and are usually triangular, as shown by Fig. 

 248. When division is about to occur, the base of the triangle 

 becomes indented, and the indentation travels towards the apex 

 in such a way as to bisect the triangle. The two halves diverge 

 from one another during the process, so that before division is 

 complete, they are looking, in some cases, in opposite directions. 



1 Rare and Remarkable Animals of Scotland, ii. 1848, p. 93. 



2 Trembley, Mem. Hist. Polypes, 1744 ; iii. Mem., p. 217. The same processes 

 are described by Baker, Employment for the Microscope, new ed. 1785, p. 311. 



