68 



I^'VEETEBEATE ANIMALS. 



than the ordinary hydrothecse. As a general rule, each capsule in 

 the Sertularida contains a number of the reproductive buds, which 

 are all borne upon a central axis which runs up the middle of the 

 capsule, and is derived from the ccenosarc. 



In one division of this group — often described as a separate order, 

 under the name of Campanularida — some points of difference are 

 observable. In the typical Sertularians the little cups or hydrothecse 

 for the polypites are placed on the sides of the branches, and they 

 are not stalked (fig. 34, a'), whilst the reproductive elements are 

 produced in fixed buds. In the Camjxinularidrx, on the other hand 

 (flg. 34, 6), the hydrothecEe 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 zooids 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 cif a 

 manubrium (fig. 36, a). The whole or- 

 ganism 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 cen- 

 tral polypite is furnished with it mouth 

 at its distal end, and the mouth opens 

 into a digestive sac. From the proximal 

 end of this stomach proceed four radiat- 

 ing 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. Fi'om the circumfer- 

 ence of the bell hang also a number of 

 delicate extensile filaments or tentacles ; 

 and the margin 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 partiallj' closed by a delicate transparent membrane or 

 shelf, the so-called " veil." Thus constituted, these beautiful little 

 beings lead an independent and locomotive existence for a longer 

 or shorter pei-iod. Ultimately, ova and sperm-cells are produced in 



Fig. 36.— Grinophore of one of 

 the CampaintlarUhi. 



