164 



ZOOLOGY 



SECT. 



But the most characteristic anatomical feature of the group is 

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

 (Figs. 112 and 113, tc) consisting of an outer layer of ectoderm 





iyy 



r 



2.Clos3ocodoi 



V, 



Fig. 112— Two Trachymedusae. ch\ c. circular canal; f/o/?. gonad ; );i/i/>. manubrium ; fwih. 

 mouth ; rad. c. radinl canal ; re. c. recurrent canal ; t. tentacle ; tc. tentacul-ocyst ; tg. tongue ; 

 vi. velum. (After Haeckel.) 



t.r 



\ 



^ ) 



Tnjh 



cc 



rdd.c 



mth 

 l.Cunarcha 



2.Polycol|>a 



Fio. 113. — Two rjarcomedusje, 2 in vortical section, gon. gonad; tunh. manubrium ; mUi, 

 mouth ; 'pr. peronium ; rad.c. radial canal ; t. tentacle ; tc. tentaculocyst ; t.r. tentacle-root ; 

 C.I. velum. (After Haeckel ) 



enclosing a central axis of endoderm cells (Fig. 114): they have, 

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

 lithites, which are always derived from the endoderm. To 



