PHYLUM CCELENTERATA 



155 



distinguish them from the lithocysts of Leptomedusae, and to mark 

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

 cysls. They may either project freely from the margin of the 

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

 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 Trachylinae are characterised by the mode 

 of origin of the tentacles. 

 In Trachymedusae, as in the 

 preceding order, they arise 

 near the edge of the um- 

 brella (Fig. 112), but in the 

 Narcomedusae they spring 

 about half-way between the 

 edge and the vertex (Fig. 

 113), and are continued, at 

 their proximal ends, into the 

 ielly of the ex-umbrella in 

 the form of " tentacle-roots " 

 (Lr). 



As to the position of the 

 reproductive organs, there 

 is the same difference be- 

 tween the two sub-orders 

 of Trachylinae as between 

 the two sub-orders of Lepto- 



linae. In the Trachymedusae the gonads (Fig. 112, gon) are 

 developed in the course of fche radial canals : in the Narcomedusae 

 (Fig. 113) they lie on the manubrium, sometimes extending into 

 the pouch-like offshoots of its cavity. 



There is always a well-developed velum, which, as in Fig. 113, 1, 

 may hang down vertically instead of taking the usual horizontal 

 position. In the Narcomedusae the manubrium is short ; in the 

 Trachymedusae it is always well developed, and is sometimes (Fig. 

 112, 2) prolonged into a long, highly contractile peduncle, having 

 its inner surface produced into a tongue-like process (tg) which 

 protrudes through the mouth. In some the gastric cavity is 

 situated in the manubrium, which in such a case is looked upon as 

 partly of the nature of a process of the sub-umbrella (pseudo- 

 manubrium). 



The simplest case of the development of Trachylinae is seen in 

 jEgirwpsis, one of the Narcomedusae. The oosperm gives rise to 

 a ciliated planula, which forms first two (Fig 115), then four 

 tentacles, and a mouth, hypostome, and stomach. The larva of 

 iEginopsis is thus a hydrula, closely resembling the corresponding 

 stage of Tubularia. After a time the tentacular region grows out, 

 carrying the tentacles with it, and becomes the umbrella of the 



Fig. 114.— .ffieinura myosura, a tentaculo* 

 cyst highly magnified, erf. ectoderm ; end. 

 endoderm ; I. lithites ; nlc. nematocysts ; 

 nc.c. group of nerve-colls. (After Haeckel). 



