OBSERVATIONS OX PRESH-WATEli POLYZOA. 



105 



(fig. 16, PL XVIEI. Lin. ej)itli.) is very thin and has the nuch-i scat- 

 tered at great intervals. I was not aljle to detect any trace of cih'a on 

 the lining epithehiim, but the rapid motion 'of the [)erigastric "fliud, 

 a'oins" toward the tii) alono- one side and coming' back ak^n«' the other 

 in the narrow tentacular cavity, indicates their existence. The 

 lumen is a little more than 0.01 mm. in diameter. 



The account, given by Yerworn, oi' the manner of junction of 

 tlie tentacles with the Io[)hophore and the tentacular membrane in 

 CrUtatella applies equally well to the species investigated by me. 

 In fig. 18, PL XVIII, I have endeavored to show diagrammatically 

 the relation of several parts at the bases of tentacles. 



Externally io the row of tentacles there is a thin membrane, 

 the tentacular membrane, 0.3-0.4 mm. in Ijreadth, formed by a, 

 duplicature of the outer layer of the lophophoral wall along its outer 

 edge. It consists of a basement membrane covered on both sides by 

 a layer of flat cells, tlie direct continuation of the outer layer. The 

 basal portion of each tentacle is joined to the tentacular membrane 

 by another narrow triangular membrane. 



Alternating with the bases of the tentacles, a series of duplicatures 

 (jn each side of the lophophoral cavity is produced in the inner layer, 

 so that if Tve were to cut across the arm and look into it, we should 

 see a series of vault-like arches. The tentacular cavity opens »nto that 

 of the lophoph(3re l^etween each two of such folds of the inner layer. 

 These folds descend almost to the floor of the lophophoral cavity, and 

 have been reckoned as part of the muscular system by Hyatt, under 

 the name of" brachial contractors," but I see no ground for regard- 

 ing them as such, since they consists simply of flat cells. 



The bases of the tentacles are not in one plane. Those on the 

 anal side near the e[)istoine are the most elevated. Tlie numl)er of 

 the tentacles is generally even, but in some individuals there is a 



