154 



ZOOLOGY 



SECT. 



ORDER 2. TRACE YLINJE. 



General Structure. The members of this order are all 

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

 except Cunina parasitica, and several species have been proved 

 to develop directly from the egg. They thus differ from the members 

 of the preceding order in the fact that no alternation of generations 

 ordinarily occurs in their life-history. 



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

 exceeding 100mm. (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 



gen 



1. Petas us 



S.CIossocodon 



FIG. 113. Two Trachymedusae. dr. c. circular canal ; gon. gonad ; mnb. mnnubrium 

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

 tg. tongue ; vl. velum. (After Haeckel.) 



the preceding order : this is well shown in Fig. 113, 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 

 in the young condition, although they may be replaced in the 

 adult by hollow tentacles. 



But the most characteristic anatomical feature of the group is 

 the structure of the sense-organs, which are club-shaped bodies 

 (Figs. 113 and 114, tc) consisting of an outer layer of ectoderm 

 enclosing a central axis of endoderm cells (Fig. 115) : they have, 

 therefore, the structure of tentacles. They contain one or more 

 lithites, which are always derived from the endoderm. To 

 distinguish them from the lithocysts of Leptomedusse, and to mark 



