XXXVlll 
THE VOYAGE OF H.M.S. CHALLENGER. 
yet in several sponges possessing a clear, transparent, collenchymatous mesoderm, 
particularly in Thenea {Thenea delicata, PI. VI. fig. 20), fine filaments may be frequently 
observed, produced from the base of the choanocytes and extending radially from the 
chambers into the surrounding matrix. Of course these are not flagella, but with the 
methods in vogue in Carter’s time they might easily be mistaken for them, and 
erroneous as one must admit Carter’s inference to have been, one cannot at the same 
time refuse a tribute of admiration to his surprising acuteness of observation. When- 
ever these filaments are observed the tubular collars of the choanocytes will be found to 
be of unusual shortness, and the flagellated chambers in which they occur of far less than 
the usual diameter, owing to the unusually close approximation of the choanocytes, the 
bases of which form an almost continuous wall. The whole appearance is suggestive of 
a contraction of the choanocytal wall under the influence of some strong stimulus, 
possibly of the alcohol into which the sponge was plunged on removal from the dredge. 
Eeturning now to the more normal chambers which are not thus contracted, close 
observation reveals the presence of lateral protoplasmic processes extending radially 
from the bases of the choanocytes and continuously uniting each of them with its 
surrounding fellows. These processes stain deeply with hsematoxylin and are probably 
contractile. If so the contraction of the flagellated chambers is probably partly brought 
by their action, partly by a shortening of the collums of the choanocytes drawing their 
bases towards the fenestrated membrane, which thus furnishes appoint d’ap>pui. It is no 
doubt owing to the contraction of the chambers that the centrifugal filaments are rendered 
visible. They may be traced from the choanocytes into continuity with surrounding 
collencytes or adjacent pinacocytes, if an epithelial surface happens to lie near enough 
(PI. VI. fig. 20). 
The Mesoderm. Collenehyma . — The mesoderm in its simplest form consists of 
gelatinous connective tissue (collenehyma), which was first described accurately and its 
true nature pointed out by F. E. Schulze. It consists of a clear transparent jelly-like 
basis which does not stain with reagents, and which is produced by the alteration or 
excretion of numerous more or less stellate irregularly branching cells (collencytes), which 
are irregularly distributed throughout it. Although spoken of as gelatinous the base 
is not gelatine-yielding, it is singularly unalterable under the action of reagents, and 
would probably repay a careful chemical examination. 
The collencytes may be best studied in Thenea muricata, since in this sponge the 
coUenchyma attains an abnormally rich development; but almost any collenchymatous 
sponge serves nearly as well. Careful observation proves that they always pass into each 
other by the confluence of their thread-like processes. 
Sarcenchyme . — By the modification of collenehyma in various directions numerous 
other tissues are produced ; one of the most important of these is sarcenchyma. Schulze 
has already pointed ‘out that in sponges with collenchymatous mesoderm, such as 
