142 



ZOOLOGY 



SECT. 



In Tubularia the oosperm develops, while still enclosed in 

 the sporosac, into a short hydrula, which, after leading a free 

 existence fora short time, fixes itself by its proximal end, buds, and 

 produces the colony. In Hydra development begins in the ovary, 

 and is complicated by the fact that the ectoderm of the morula 

 gives rise to a sort of protective shell : in this condition the 

 embryo is set free, and, after a period of rest, develops into the 

 adult form. 



ORDER 2. TRACHYLIX^E 



General Structure. The members of this order are all 

 medusae : no zoophyte stage is certainly known in any of them, and 

 several species have been proved to develop directly from the egg. 



FIG. 99. Two Trachy medusae, dr. c. circular canal; rion. gonad ; mnl>. inniml>rium ; mth. 

 mouth ; rad. c. radial canal ; re. c. recurrent canal ; t. tentacle ; tc. tentaculocyst ; ty. torigue ; 

 ill. velum. (After Haeckel.) 



They thus differ from the members of the preceding order in the 

 fact that there is no alternation of generations in their life- 

 history. 



Most species are of small or moderate size, the largest not 

 exceeding 100 mm. (4 inches) in diameter. The gelatinous tissue 

 or mesoglcea of the ex-umbrella is usually well developed, giving 

 the medusa a more solid appearance than the delicate jelly-fish of 

 the preceding order : this is well shown in Fig. 99, in which the 

 apical region of the umbrella has a comparatively immense thick- 

 ness. The tentacles are also stiff and strong, and are always solid 



