DIVISIONS OF THE HYDROZOA. 7t 
ectoderm and endoderm of the manubrium. From the resem- 
blance of these buds in anatomical structure to the so-called 
sea-jellies or Medusa, they are usually spoken of as “ medusi- 
form gonophores,” or simply as “medusoids.” In this form, 
however, though highly organised, the buds never become de- 
tached from the parent colony. 3. Buds which become devel- 
oped into bell-shaped medusiform bodies exactly similar in 
structure to the last, but detached to lead an independent 
existence. These free-swimming medusiform gonophores are 
anatomically indistinguishable from ordinary Meduse; and it 
is now known that most, if not all, of the so-called “naked- 
eyed” Medus@, are really the detached generative buds of 
other orders of Hydrozoa. The special elements of reproduc- 
tion are developed in these detached buds, but the resulting 
embryos are not developed into Medus@, such as produced the 
ova and sperm-cells, but straightway grow up into the plant- 
like sexless colony, from which the medusiform gonophores 
were originally budded forth. In these cases, therefore, the 
endividual Hydroid consists of a fixed, rooted colony or tro- 
phosome, producing fresh zodids by a process of budding, but 
incapable of producing the essential elements of reproduction, 
together with a free and independent series of generative 
buds or gonosome, in which the elements of reproduction are 
developed. 
ORDER III. SERTULARIDA.—In this order of the flydroida 
we have the most familiar and best known of all our zoophytes 
—namely, the sea-firs and their allies. The horny plant-like 
polyparies of the Sertudarida are familiar to every visitor at the 
sea-side, and by those unacquainted with their true nature they 
are almost universally set down as sea-weeds. The Sertularida 
are very Closely allied to the compound forms of the Corynida, 
resembling them in being rooted, plant-like colonies, composed 
of a number of similar polypites or zodids, produced by budding 
from a primitive zdoid. As in the Tubularians amongst the 
Corynida, the whole coenosarc is enveloped ina horny or chiti- 
nous envelope or polypary (fig. 20, a), and this is the structure 
which is most familiarly known to sea-side observers. The 
Sertularida, however, are distinguished from the Corynida by 
two points. Firstly, none of the Sertularida are simple, but 
are all compound, consisting of more or less numerous poly- 
pites, united by a branched ccenosarc. Secondly, the polypary 
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