IV 



PHYLUM CCELENTERATA 



155 



the fact that they are modified tentacles, they are called tentaculo- 

 ci/sts. They may either project freely from the margin of the 

 umbrella, or may become enclosed in a pouch-like growth of 



rad.c 



mth 

 l.Cunarcha 



2.Polycol|ia 



ccl 



Fit;. 114. Two Narcomedusrr, 2 in vertical section, (ion. gonad ; mnb. mamibrium ; mth. 

 mouth : pr. peronium ; ratl. c. radial canal ; t. tentacle ; tc. tentaculocyst ; t.r. tentacle- 

 root ; (7. velum. (After Haeckel.) 



ectoderm and more or less sunk in the tissue of the umbrella. 

 Eyes occur in some, and are always of simple structure. 



The two sub-orders of Trachylinse are characterised by the mode 

 of origin of the tentacles. In Trachymedusae, as in the preceding 

 order, they arise near the edge of the umbrella (Fig. 113), but in the 

 Narcomedusse they spring about halfway between the edge and 

 the vertex (Fig. 114), and are 

 continued, at their proximal 

 ends, into the jelly of the ex- 

 umbrella in the form of ' ten- 

 tacle-roots " (t.r). 



As to the position of the re- 

 productive organs, there is 

 the same difference between the 

 two sub-orders of Trachylinae as 

 between the two sub-orders of 

 Leptolinse. In the Trachy- 

 medusae the gonads (Fig. 113, 

 gon) are developed in the course 

 of the radial canals : in the 

 Narcomedusae (Fig. 114) they lie 



,-[ i FIG. 115. -ffiginura myosura, a tentacnlo- 



011 the mamibrium, sometimes cyst highly magnified tct. ectoderm ; end. 



extending into the pouch-like 

 offshoots of its cavity. 



There is always a well-developed velum, which, as in Fig. 114, 7, 

 may hang down vertically instead of taking the usual horizontal 



-e/icf 



endoderm ; /. lithites ; ntc. nematocysts ; 



n >.<. siroup of nerve-cells. (After Haerkel.) 



