154 



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 



FIG. 112. Two Trachy medusae, dr. c. circular canal ; (ton. gonad ; mnb. maimbrium ; mth. 

 mouth ; rod. c. radial canal ; re. c. recurrent canal ; t. tentacle ; tc. tentaculocyst ; tg. tongue ; 

 vL velum. (After Haeckel.) 



rad.c 



ntth 

 l.Cunarcha 



2.Polycol|>a 



FIG. 113. Two Narcomedus JC. 2 in vertical section, gon. gonad ; mnb. manubrium ; mth. 

 mouth; pr. peronium ; rnd.c. radial canal ; t. tentacle ; tc. tentaculocyst ; t.r. tentacle-root ; 

 v.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 



