HYDROZOA. 237 



The auditory organs are of two types, both beginning as free organs 

 and receiving their highest development as closed vesicles (otocysts). One 

 type, the tentacular organs, occur in the Trachymedusae, the other, or 

 velar organ, in the Leptomedusae. The tentacular organs are modified 

 tentacles, the entodermal axis of which forms the otoliths and the 

 ectodermal covering the sense cells. In the ^Eginidae (Fig. 180, A) the 

 club-like tentacles, seated on an auditory cushion, project freely into the 

 water; in the Trachynemidse (Fig. 180, B) they are partially transformed 

 into vesicles by the upgrowth of epithelium, and in the Geryonidae (Fig. 

 180, C) they are completely enclosed arid are sunk in the jelly of the bell. 

 The velar organs of the Leptomedusa3 are placed on the subumbrellar sur- 

 face of the velum. They may be either simple pits (Fig. 180, J?), or the 

 mouths of the pits may close (Fig. 180, Z>). In these both sense cells and 

 otoliths are ectodermal. Eyes and otocysts occur in different forms, a 

 fact which formerly lead to a division of medusae into ocellate and vesicu- 

 late groups. 



While polyps and medusae apparently differ so greatly from each 

 other, their morphology shows that the medusae are only highly 

 modified polyps adapted to a swimming life. The long axis of 

 the polyp has been greatly shortened (fig. 181) and the cylindrical 



FIG. 181. Diagram of sections of (A) a polyp and (B) a medusa, ek, ectoderm; ck', of 

 exumbrella; efc 2 , of subumbrella; ek\ of manubrium; e7, endoderm (cathamnal) 

 layer arising from obliteration of digestive space ; en, entoderm ; r, ring canal ; 

 s, subumbrella ; t, tentacles; V, velum ; x, supporting layer (gelatinous in B). 



body developed into a disc; the mesoglcea of column and disc thick- 

 ened to a conspicuous layer of jelly; while manubrial cavity, 

 radial and ring canals are to be interpreted as remnants of the 

 large gastrovascular space of the polyp, obliterated in part by the 

 pressure of the mesogloea. To the parts thus formed only the 

 yelum and sense organs are added. 



This comparison of medusa with polyp is of importance in 

 understanding the development, which usually is complicated by 

 an alternation of generations. From the eggs of the medusae a 

 small ciliated embryo (planula) escapes, which becomes attached, 



