68 



INVERTEBRATE ANIMALS. 



" ovarian vesicles " or " capsules." These reproductive buds 

 are enclosed in horny cups or receptacles, often of a very 

 beautiful shape, and much larger in size than the ordinary 

 hydrothecae (Fig. 17, , a'}. Each bud may be compared to a 

 polypite destitute of a mouth and tentacles, being composed 

 of a protuberance of the ectoderm and endoderm, containing 

 a prolongation from the general cavity of the ccenosarc. The 

 essential elements of reproduction are developed between the 

 ectoderm and endoderm of the bud, and the resulting embryo 

 is finally liberated as a little oval body covered with cilia, 

 with which it swims freely about, until it meets with a suitable 

 locality, when it fixes itself, loses its cilia, and by budding 

 soon develops another colony. 



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 hydro thecas for the polypites are placed on the 

 sides of the branches, and they are not stalked (Fig. 17, a'), 

 while the reproductive elements are pro- 

 duced in fixed buds. In the Campanu- 

 larida, on the other hand (Fig. 17, #), 

 the hydrothecae are supported upon 

 stalks, and are placed at the ends of 

 the branches, while the generative buds 

 are usually detached to lead an inde- 

 pendent existence. In these forms the 

 reproductive zooids or gonophores start 

 as simple buds ; but they become grad- 

 ually 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 sus- 

 pended a modified polypite, in the form 

 of a manubrium (Fig. 18). The whole 

 organism swims gayly through the 

 water, propelled by the contractions of 

 the bell or disk ; and no one would sus- 

 pect now that it was in any way related 

 to the fixed, plant-like zoophyte from 



na. i8.-Gono P hore of one of which it was originally budded" off. The 

 the Campanularida. central polypite is furnished with a 

 mouth at its distal end, and the mouth 

 opens into a digestive sac. From the upper end of this stomach 



