IV 



PHYLUM CCELENTERATA 



143 



the young condition, although they may be replaced in the 

 lult by hollow tentacles. 



But the most characteristic anatomical feature of the group is 

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



rad.c 



nith 

 t.Cunarcha 



2.Polycolf>a 



ect 



FIG. 100. Two Narcomedusse, 2 in vertical section, gon. gonad ; mnb. manubrium ; mth. 

 mouth ; pr. peronium ; t. tentacle ; tc. tentaculocyst ; t.r. tentacle-root ; vl. velum. (After 

 Haeckel.) 



(Figs. 99 and 100, tc) consisting of an outer layer of ectoderm 

 enclosing a central axis of endoderm cells (Fig. 101) : 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 Leptornedusae, 

 and to mark the fact that 

 they are modified tentacles, 

 they are called tentaculocysts. 

 They may either project 

 freely from the margin of the 

 umbrella, or may become en- 

 closed in a pouch-like growth 

 of ectoderm and more or less 

 sunk in the tissue of the 

 umbrella. 



The two sub-orders of 

 Trachylinae are characterised 

 by v the mode of origin of 

 the tentacles. In Trachy- 

 medusse, as in the preceding 



order, they arise near the edge of the umbrella (Fig. 99), but in 

 the Narcomedusse they spring about half-way between the edge 

 and the vertex (Fig. 100), and are continued, at their proximal 



end 



ntc 



FIG. 101. -ffiginura myosura, a tentaculo- 

 cyst highly magnified, ect. ectoderm ; end. 

 endoderm ; I. lithites ; ntc. nematocysts ; 

 nv.c. group of nerve-cells. (After Haeckel). 



