EEPORT ON THE TETEACTINELLIDA. 
xxxvii 
» 
the flagellated chamber (PL VIII. fig. 20 ; PI. XII. fig. 28). Looked at face on this 
membrane has the appearance of being fenestrated by regularly distributed round or 
oval openings, which represent the openings of the tubular collars of the choanocytes 
(PL VIII. fig. 20). Sometimes (PL XXXIII. fig. 11) a small cell with a distinct nucleus 
is present in the lumen of one of these fenestrae ; this appears to belong to one of the 
choanocytes, which has been torn from its attachment to the chamber-wall and pulled by 
the contraction of the collar to a level with the membrane. The physiological meaning 
of this structure is not obvious ; that it is not an artificial product appears from the 
constancy with which it is present in certain groups and not in others ; I have never 
seen more than doubtful signs of it in the Monaxonids, it certainly does not occur in the 
Suberitidae nor in the Tethyidse, and it may possibly prove to be characteristic of the 
Tetractinellida only. 
Since the fenestrated membrane stretches across the flagellated chamber transversely 
there must be an aperture or apertures in it for the passage of water from the prosodus 
to the aphodus, though I have never succeeded in finding any ; it is possible that the 
apertures have the form of pores no larger than the lumen of the choanocytal collars, 
and in this case they would be ver}^ difficult to distinguish. It is also a curious fact 
that I have never yet seen the flagella of the concrescent choanocytes,. though I have 
never failed to find them in the case of choanocytes which are not concrescent. It 
might be explained on the supposition that the flagella are retracted in the former 
case ; but that naturally leads to the inquiry as to why they are not retracted in the 
latter. 
The concrescent choanocytes are always restricted to the prosodal face of the 
flagellated chamber, extending sometimes quite up to the aphodus, sometimes ceasing 
halfway across the length of the chamber from it, and since they are always directed with 
the collars pointing towards the aphodal end of the chamber, the fenestrated membrane is 
constantly concave towards the aphodus and thus it is easy to distinguish the aphodal 
from the prosodal pole of the chamber, simply by an inspection of the fenestrated 
membrane. The collars of those choanocytes which lie nearest the aphodus are always 
shorter than of those which lie further away, a fact already noticed by von Lendenfeld in 
Aplysilla violacea, Lend.^ This reduction in length of the choanocytes as they approach 
the prosodal pole renders the fenestrated membrane more concave than it would 
otherwise be. 
Carter, after having rightly described the structure and mechanism of the flagellated 
chamber, subsequently represented it as consisting of choanocytes with the flagella 
turned outwards, i.e., in the light of our present knowledge into the mesoderm. This is 
made the subject of scoff by Professor Haeckel, who is nothing if not logical, and 
unquestionably the presence of flagella in such a position would be a little surprising, 
^ Zeitschr. f. wiss. Zool., Bd. xxxviii. p. 260, pi. xi. fig. 13. 
