CLASS VI. TUNICATA: OEDER 2. ASCIDI^. 631 
These animals comprise several extensive groups or families. The Compound Ascidians, or 
Botryllidcey are united together by the coalescence of their mantles, so as to form a leathery or 
gelatinous mass, usually attached to stones 
or sea-weed; in this the separate animals 
are imbedded round a common canal, 
many of them being adorned with beau- 
tiful colors, and these masses are of an 
almost infinite variety of forms. In some 
cases, the polypidom, or as it is gen- 
erally called, the Polyzoarium, is a leaf- 
like, expanded calcareous mass, attached 
to a rock, the cells of the separate animals 
opening by hundreds on one surface ov\j. 
The ClaveUinidce, or Social Ascidians, 
are imited by means of a sort of creeping 
stem, which runs along the surface of svd)- 
marine objects, and gives rise at inter\ als 
to short footstalks, at the extremity of 
which the animals are supported. The 
separate animals are produced by gerama- 
coKPouNB ASCIDIANS, MAGNL-iED. ^-^^^ f^.^^^ crccpiug stems, which run 
in various directions from the base of the original founder of the colony. 
The family of Ascidiidce, or Simple Ascidians, is composed of animals which live separately, 
attached by the base to submarine bodies. They usually form shapeless masses of a cartilaginous 
texture, often of considerable size, w^hicb occur in great abund- 
ance in shallow water. A few species are eaten in some coun- 
tries. 
The fourth family, the Pyrosomatidce, agree closely with 
the salpse in the general arrangement of their organs. The 
colonies of these singular creatures are in the form of a cartila- 
ginous tube, open at one end. In the walls of this tube, formed 
by the coalescence of the mantles of the animals composing- 
it, the bodies of the Ascidians are separately imbedded, the 
branchial chamber of each passing completely through the wall 
from its outer to its inner siirface. These animals are found in the seas of warm climates, w^here 
they float along in an upright position, but apparently possess no actual locomotive power. 
Like all the Tunicata, they are luminous in the dark, and in fact appear to possess this faculty 
in a greater degree than any other 
members of the class. Their cvlin- 
drical form, upright position, and con* 
siderable size, which often exceeds a 
foot, render them exceedingly beauti- 
ful objects at night ; and they have 
been described as resembling little 
columns of fire. Mr. George Bennett describes a scene which he witnessed at night in the Indian 
Ocean, in which a ship passed through a shoal of these animals a mile in width, shining like a 
liquid mass of pale greenish light. The scene was as novel as it was interesting. 
SOCIAL ASCIDIANS, MAGNIFIED. 
PYliOSOMA. 
The term Bryozoa is derived from the Greek, hruon, moss, and zoon, an animal, and is descrip- 
tive of the animals of this class, which always grow together upon a common stock, in the same 
manner as the compound polypi, with which they were formerly arranged. Each ^nimal resides 
