Introduction to Animal Alorphology. 87 



12, B & C). Many zooids undergo further change, and 

 losing the ectotheca, the free pole of the mesotheca 

 opens, forming a bell, with the spadix (now called 

 manubrium) enclosed as a clapper (Fig. 12, D), and its 

 cavity, opening at its extremity, forms a mouth (Fig. 

 12, D, ///.) communicating with the stomach or cavity 

 of the manubrium, and through it with the radial 

 canals (Fig. 12, D, f.c), whose peripheric ends become 

 united by a circular canal (E, c.) around the margin of 

 the mouth of the bell or codonostome. These tubes 

 are called the gastro-vascular canals.* From the free 

 •edge of the codonostome, tentacles project (/.), and an 

 in-growing muscular horizontal velum (f.) narrows the 

 aperture. This form of zooid is called phanerocodonic, 

 and it usually loses the pedicle joining it to the 

 coenosarc, becomes free, and is called a planoblast or 

 medusa. t Ova or spermatozoa may develop under 

 the ectodermal layer of its manubrium (Fig. 12, D, s\ 

 and it is then called a gonocheme ; but often the 

 medusa produces along its radial canals, within the 

 cavity of the bell, a second race of zooids, usually 

 sporosacs (Fig. 12, E, ^.), in which the sexual products 

 are formed. Such medusae are called blastochemes, 

 and the second race are either saccular, or, as in 

 Tima, a long wavy tube along the entire radial canal. 

 To any of the zooids in which ova or spermatozoa are 

 formed, the name gonophore has been given. A 

 columnar zooid supporting gonophores along its side 

 is named a blastostyle. The manubrium of a free 

 medusa is homologous with the hypostome of the 



* A circular lip below the stomach can, by contracting, temporarily 

 shut off these canals from the stomach. 



t Most of the small floating umbrella-hke Jelly fishes, abundant in our 

 seas, usually from the size of a pea to that of a walnut, belong to this class. 



