1 THE VOYAGE OF H.M.S. CHALLENGER. 



as in the closely allied Nauphanta (Pis. XXVII., XXVIII.), eight adradial tentacles (fig. B, 

 to) are developed between the eight principal. In the majority of the Medusae the 

 number of the tentacles increases with age, as new tentacles are formed later between the 

 original four or eight. This increase takes place according to fixed laws, which vary 

 in the different principal groups. The ontogenetic series in the appearance of the 

 different orders of tentacles, allows us to conclude that there is a corresponding phylo- 

 genetic progression. In contrast to the eight principal tentacles (four perradial and four 

 interradial) all the others which appear between them later may be termed succursal. 

 This distinction is important because the eight principal tentacles give rise to numerous 

 transformations and progressive formations. By retrograde formation of two opposite 

 perradial tentacles, dissonemal Medusa?, e.g., Thamnostylus dinema (PI. I.) often arise from 

 tetranemal. Such Medusa? are found among many groups of the Craspedota?, but not 

 among the Acraspeda?. The remains of the retrograded tentacles usually persist as 

 bulbs of the umbrella margin between the two opposite permanent tentacles. On the 

 other hand, we very rarely find only a single tentacle in the developed Medusa?, the 

 three others having undergone retrograde formation ; the Euphysida?, a small sub-family 

 of the Codonida? (System, taf. ii.) are mononemal Medusa?. The Amaltheida? among the 

 Craspedota?, and the Ehizostoma? among the Acraspeda?, are distinguished by complete 

 loss of all the tentacles. 



§ 78. Form and structure of the tentacles. In most Medusa? the tentacles are long, 

 cylindrical filaments, more rarely flattened like a ribbon. They are usually thicker 

 at the base, but pointed conically towards the end, more rarely swollen like a club. 

 They are almost always simple and unbranched ; only a single family of the Craspedota?, 

 the Cladonemida?, are distinguished by branched or composite tentacles (System, taf. vii.) ; 

 these are sometimes branched dichotomously, sometimes beset with " secondary filaments " 

 (semi-pinnated) as in the Siphonophora and Ctenophora. The structure of the tentacles 

 is essentially the same in all Medusa?. They are composed of the same four essential layers 

 of tissue (or " secondary germinal layers") as the umbrella itself, namely: — (l) the outer 

 epithehum of the endoderm ; (2) the muscular plate underlying it, formed of longitudinal 

 fibres ; (3) the structureless, elastic supporting plate ; (4) the inner cellular axis of the 

 endoderm. We distinguish two principal forms, sobd tentacles and hollow tentacles ; 

 both often appear in closely albed Medusa?, sometimes beside one another in one and the 

 same species (Geryonida?). They are principally to be distinguished by the comportment 

 of the endodermal axis. The solid tentacles are usually stiffer and shorter, less extensible 

 and flexible ; they are chiefly found in the Trachylina? (Trachomedusa? and Narcomedusa?) 

 and also in the oldest forms of the Acraspeda? (Stauromedusa? and Cannostoma?). Their 

 cylindrical endodermal axis usually consists of a single row of discoid chordal cells, lying 

 the one above the other like the coins in a rouleau of sovereigns (PI. VI. fig. 17 ; PI. 

 XIII. figs. 5, 6, &c. ; PI. XV.). They are more rarely arranged in several layers (PI. IV. 



