532 
MOLI^USCA. 
in a separate cell, witliin wliicli it can usually retract itself entirely; the cells are sometimes soft 
and flexible, sometimes liorny, and sometimes calcareous. They frequently stand upon short foot- 
stalks, rising from a tubular stock, which creeps over the surface of stones and aquatic plants. 
In other cases the cells are sessile, forming a crust upon submarine objects, while in others the 
colony is attached only by its base, with the opposite extremity floating freely in the water. In 
these the stock is more or less branched and often leaf-like. They possess ciliated tentacula 
placed at the anterior extremity of the body. The colonies are increased by gemmiparous repro- 
duction. The class is divided into two orders, the Loyihopoda and Infundibulata* 
ORDER 1. LOPHOPODA. 
In this order, composed of inhabitants of fresh water, the tentacles, which are more numerous 
than in the preceding, are placed upon a pair of 
long arms, which spring from the sides of the 
upper extremity of the animal, and usually de- 
scribe somewhat the form of a horse-shoe, as is 
seen in the PlumateUa. Their cells are usually 
of a leathery texture, nearly transparent, and 
usually spring from a root-stock of similar con- 
sistence, which creeps along upon the surface 
of stones or aquatic plants, in which situations 
these animals are not uncommon. In some 
species, however, as in the Cristatella, the 
polypidom floats freely in the water, and is of a 
gelatinous consistency ; in these the animals 
composing each colony are usually three or four 
in number. 
-'31 \. 
ORDER 2. INFUNDIBULATA. 
This order derives its name from infundihulum, a funnel. It includes a great number" of genera, 
presenting an infinite variety of structure. The commoii Flus- 
trce, or Sea-Mats, are flat and foliaceous in their form^ pre- 
senting a considerable resemblance in appearance to pale-brown 
sea-weeds, with which they are in fact generally confounded 
by sea-side visitors. But when carefully examined, these 
leaf-like bodies will be found to consist of a multitude of 
small horny cells, opening at the surface ; and from each of 
these, when the polypidom is placed alive in a vessel of 
sea-water, the little creatures may be seen protruding their 
tentacles. Many nearly allied species grow upon the fronds 
of sea-weeds, over which they spread like a thin coating of 
gauze, composed of similar cells, opening of course only on one 
side. Others are found incrusting stones and other submarine 
bodies with a cellular calcareous mass. In many species the 
cells are arranged so as to form a more or less threadlike, 
branching polypidom; while others are furnished with a creeping root, from which the cells rise 
by stems of greater or less length. All the Infundibulata are marine animals. 
PLCSTRA aviculaeia; natural size. 
* The Tunicata and Bryozoa — the Pyrozoa of some authors — were formerly arranged with the Polypi, though now- 
placed with the Mollusca ; they are, however, so peculiar in certain respects as to constitute with some naturalists 
a separate division under the name of Molluscoides. 
