ESCHARID.E. 333 



of these species, and indeed are very rarely to be seen on some of 

 them ; and when present it is only upon some of the cells. Krohn 

 has classified them under three different forms : (1) Those which 

 have the figure of the crab's arms; (2) those which resemble 

 pincers ; and (3) those which are formed like bristles or hairs. 

 The first kind, or proper "bird's head" process, is found on Cel- 

 lularia avicularia and ciliata, and on Flustra avicularis and 

 murrayana ; and the second, which only differs from the first in the 

 absence of a moveable pedicle, on the Cellularia scruposa and Rete- 

 pora cellulosa. Both kinds are always placed on the outer edge of 

 the cell which produces them, and the first are articulated to it, so 

 that they move freely up and down on the basal joint. Besides their 

 pendulum-like motion, they can also shut and close their "bills" at 

 pleasure ; and all these motions are affected by appropriate muscles. 

 Van Beneden has traced their growth step by step, and studied all 

 the phases of their development, but without obtaining any clue to 

 their use or purpose in the economy of the polype. Pallas suspected 

 that they had some connection with the reproductive function, but 

 this is very improbable. Dr. Reid has thrown out a conjecture that 

 they may assist in circulating water along the canals in the different 

 processes of the cell. 



To the third form belong the more or less elongated bristles which 

 are observed on the external margin of the cells, and every cell of 

 the polypidom is equally furnished with them. They are found in 

 Cellularia scruposa, reptans and Hookeri, and are minutely described 

 by Van Beneden and Dr. Reid. A figure copied from the former will 

 give a better idea of the structure than a more elaborate description. 

 (Fig. 66.) Dr. Reid gives this short summary of his more detailed 

 account : " At the anterior part of the outer side of each cell in the 



Fig. 66. 



