74 INVERTEBRATE ANIMALS. 
reproductive elements are produced in fixed buds. In the 
Campanularida, on the other hand (fig. 20, 4), the hydrothece 
are supported upon stalks, and are placed at the ends of the 
branches, whilst the generative buds are usually detached to 
lead an independent existence. In these forms the reproductive 
zodids or gonophores start as simple buds; but they become 
gradually developed into free-swimming medusoids, such as 
have been before alluded to. Each medusoid consists of a 
little transparent glassy bell, from 
the under surface of which there is 
suspended a modified polypite, in 
the form of a manubrium (fig. 22). 
The whole organism swims gaily 
through the water, propelled by the 
contractions of the bell or disc ; and 
no one would suspect now that it 
was in any way related to the fixed 
plant-like zoophyte from which it 
was originally budded off. The 
central polypite is furnished with a’ 
mouth at its distal end, and the 
mouth opens into a digestive sac. 
From the proximal end of this 
stomach proceed four radiating 
canals which extend to the margins 
of the bell, where they all open 
into a circular vessel which runs 
round the mouth of the bell. From 
the circumference of the bell hang 
Fig. 22.—Gonophore of one of also a number of delicate externsile 
the Campanularida. filaments or tentacles ; and the mar- 
gin is further adorned with a series of brightly-coloured spots, 
which are probably rudimentary organs of vision and hearing. 
The mouth of the bell is partially closed by a delicate trans- 
parent membrane or shelf, the so-called “ veil.” Thus constituted, 
these beautiful little beings lead an independent and locomotive 
existence for a longer or shorter period. Ultimately ova and 
sperm-cells are produced in special organs, which are developed 
in the course of the radiating canals of the disc. The resulting’ 
embryos are minute free-swimming bodies, covered with cilia, 
which finally fix themselves, and develop into the plant-like 
colonies from which the medusoids were derived. 
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