THE SERTULAKIAN ZOOPHYTES OF OUR SHORES. 
229 
it propels it through the water. From the margin of the bell 
depend a number of extensile tentacles, and a delicate membrane 
partially closes it below. The central body is furnished with a 
mouth at its free extremity, and contains a digestive cavity, 
which communicates with four canals that traverse the sub- 
stance of the bell and empty themselves into a circular vessel 
running round its margin. On detaching themselves from the 
colony, and emerging from the capsule, these vagrant members, 
driven by their contractile swimming-bells, dance gaily through 
the water, and exhibit, on a casual inspection, not a trace of 
their affinity v/ith the rooted and vegetative being from which 
they have but lately parted. 
Their structure is that of the jelly-fish, and for a long time 
they were separated, in the systems of zoology, from their own 
kindred, and banished to a distinct class. When their escape 
from the capsule of the zoophyte was first observed, they were 
regarded as the embryo, and it was a marvel and mystery 
that the child should be so totally unlike its parent. The 
marvel has now vanished, and the mystery is solved ; but the 
simple facts, as interpreted by science, have all the interest of a 
romance. It was on a false reading of these facts amongst others 
that Steenstrup’s ingenious theory of ‘‘ the alternation of gene- 
rations” was founded; a theory that captivated by its novelty, 
while the imaginative dress in which it was clothed gave it an 
additional charm ; but which rests on a complete misconception 
of the Hydroid economy.* 
The locomotive medusiform member of the Hydroid colony, 
which discharges the sexual functions, is, in fact, a swimming 
polypite — a modification of the structure that appears in the 
alimentary zooid, and not built upon a different type. It is 
essentiall}^ a polypite, with its tentacles united by a membran- 
ous w^eb, which acts as a float and propulsive organ. A few 
elements are superadded to those that are present in the 
ordinary polyp ites ; ocelli and other rudimentary organs of sense 
are sometimes set along the margin of the bell (Plate XLV. 
fig. 5e), and a circular vessel runs round it, which combines with 
the radiating canals to form a simple circulatory system ; but 
the basis of structure is the same in both. The medusoid, or 
locomotive sexual zooid, is a polypite adapted to a free exist- 
ence, but effectually disguised by its adaptive dress. 
it may be remarked that the superficial dissimilarity between 
the fixed and floating members of the Hydroid colony is not 
greater than that between the leaf and flower-bud of the plant, 
with which they in some measure correspond. 
* Dr. Carpenter was the first (in a remarkable paper published in ISIS) 
to challenge this theory, and to appreciate the real significance of the facts 
on which it is based. 
