OUR PRESS-WATER POLYZOA. 
307 
to the order called Gymnolcemcita which, comprises nearly all 
the Marine Polyzoa. The space included between the endocyst 
and the alim entary canal is termed the perigastric space. 
Let us now take a specimen of Cristatella Mucedo, and 
observe those other details of structure or economy which 
require consideration. You cannot select a better specimen 
for study than this beautiful species, which, unlike some of the 
other Polyzoa, is not at all particular, nor shy of being treated 
roughly. The Polypides protrude themselves most good-temper- 
edly from the orifices of the cells, and if a sudden shock to 
their rudimentary nervous system causes them to contract 
within their chambers, they will pop out again almost imme- 
diately; while some of the Plumatellidce are occasionally very 
shy in making them appearance. Sometimes you may be obliged 
to force the Polypides out of their cells in order to see whether 
they are alive or not, a fact which is easily ascertained by the 
ciliary motions of the tentacula. 
The engraving (Plate XIII., fig. 3) is copied from a drawing, 
made by a friend, of a living specimen of Cristatella taken last 
July, and is magnified about two and a half diameters. The 
colony, which varies in length from between half an inch to 
about two inches, is of a long oval shape, yellowdsh in colour, 
with large dark spots, which are the statoblasts imbedded in 
the caenaecium, and with smaller ones which are pellets of faecal 
matter. The upper sui’face is convex, and around the margin 
there are three regular concentric series of Polypides alternating 
one with another, while the central portion of the caenaecium is 
unoccupied. This arrangement is not seen in the drawing, 
because the portrait was taken soon after the specimen had 
been taken out of the collecting bottle, and before it had time 
to assume its normal position. The under-surface consists of 
a flattened muscular disc, by means of which the little colony 
attaches itself to submerged weeds, or moves along from place 
to place. The tentacula are very numerous, from seventy-five 
to eighty in number ; they are hollow, and communicate with 
the lophophore and the perigastric space. The cilia, which are 
always seen to vibrate in an upward direction on one side of a 
tentacle, and downwards on the other, answer the double pur- 
pose of respiration and of supplying, by the currents they 
produce, food for the little inhabitants of the cell, which food 
consists chiefly of Desmideae and the spores of Algae. 
The lophophore, in most of the Fresh-water Polyzoa, presents 
the form of a crescent, or a horse-shoe ;f in Fredericella this 
* From gymnos, “ naked,” and lavtna. 
t Whence the term, Hippocrepia ( hippos , “ a horse,” and crepis, “a shoe), 
proposed by M. Gervais to designate the Fresh-water as distinct from the 
Marine Polyzoa, to which latter group he applies the term Infundibulata 
(funnel-shaped). 
