88 MANUAL OF ZOOLOGY. 



chief difference consists in the fact that the hydrothecae of the 

 Campamilarida with their contained polypites are supported 

 upon conspicuous stalks, thus being terminal in position (fig. 

 17, b)', whilst in the Sertularida they are sessile or subsessile, 

 and are placed laterally upon the branchlets. The gonophores 

 also in the Campanularida are usually detached as free-swim- 

 ming medusoids, whereas they remain permanently attached 

 in the Sertularians. Each medusoid consists of a little trans- 

 parent glassy bell, from the under surface of which there is 

 suspended a modified polypite, in the form of a " manubrium" 

 (fig. 15). The whole organism swims gaily through the water, 

 propelled by the contractions of the bell or disc (gonocalyx) ; 

 and no one would now suspect 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 circumference of the disc, where 

 they all open into a single circular vessel surrounding the 

 mouth of the bell. From the margins of the disc hang, also, a 

 number of delicate extensile filaments or tentacles ; and the 

 circumference is still further adorned with a series of brightly- 

 coloured spots, which are probably organs of sense. The 

 mouth of the bell is partially closed by a delicate transparent 

 membrane or shelf, the so-called " veil." Thus constituted, 

 these beautiful little beings lead an independent and locomo- 

 tive existence for a longer or shorter period. Ultimately, the 

 essential elements of reproduction are developed in special 

 organs, situated in the course of the radiating canals of the 

 disc. The resulting embryos are ciliated and free-swimming, 

 but ultimately fix themselves, and develop into the plant-like 

 colony from which fresh medusoids may be budded off. The 

 ova in the medusiform gonophores are usually developed in 

 the course of the gonocalycine canals, and not between the 

 ectoderm and endoderm of the manubrium, as is the case in 

 the Corynida. Examples of the order are Campanularia, 

 Laomedea, &c. The distinctions between the Sertularida and 

 Campanularida are certainly insufficient to justify their being 

 placed in separate orders. If united together, it would pro- 

 bably be best to adopt the name Thecaphora (Hincks) for the 

 order, and to employ the names Sertularida and Campanularida 

 for the sub-orders. 



