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



can operate like a helm in the motion of swimming. From the ontogeny of the 

 Acraspedae (Amelia) it is probable that their marginal lobes have really originated 

 phylogenetically from tentacles. A three-cleft Scyphostoma tentacle can have given rise 

 in the Ephyra formation to a rhopabum and the two " ocular lobes" enclosing it. The 

 number of the lobes varies greatly. In the Acraspedae there are at least eight adradial 

 marginal lobes (fig. D, Pis. XVL-XVIL). In place of these, however, we usually find 

 sixteen subradial (Pis. XVIII.-XXVIIL), and their secondary number is often con- 

 siderably increased (Pis. XXX.-XXXIL). 



§ 70. Velarium of the Cubomedusae. Whilst in most Acraspedae the marginal lobes 

 project freely at the umbrella margin, alternating with the tentacles and rhopalia, the 

 Craspedotae are distinguished by the marginal lobes being fused together or connected by a 

 thin intermembrane, like a swimming membrane (PI. XXXVI. ; System, taf. xxv.-xxvi.). 

 In this way a muscular, broad, thin marginal membrane is formed, which strongly 

 resembles the velum of the Craspedotae, and has hitherto been generally considered 

 homologous with it : it differs completely from the latter, however, both in its origin and 

 its finer construction, and is therefore more appropriately termed velarium. The true 

 velum of the Craspedotae, and the velarium confounded with it of the Cubomedusae have 

 arisen quite independently of one another and in a different manner ; the two bear a 

 completely different morphological relation to the umbrella margin and to its nerve ring. 

 The velarium of the Cubomedusae is usually traversed by canals (distal processes of the 

 coronal pouches), (PL XXVI. fig. 8), whilst this is never the case in the velum of the 

 Craspedotae. Moreover, the velarium in most Cubomedusae is suspended by four 

 perradial "frenula" (or gelatinous ridges of the subumbrella. Comp. above and 

 PI. XXVI. figs. 2, 8, vf). The velarium differs in the two families of the Cubomedusae, 

 inasmuch as it is composed of eight adradial marginal lobes in the Charybdeidae, but of 

 sixteen subradial marginal lobes in the Chirodropidae. The marginal lobes are fused to 

 a velarium in the same way, but not so apparently in many Discomedusae, such as the 

 Ehizostomae. It is very broad, for example, in Drymonema (Pis. XXX., XXXL). A 

 narrow circular border of the umbrella margin, which in some Discomedusae is developed 

 below the corner of tentacles (Aurebdae, System, taf. xxxiii. fig. 8, vet) differs both from 

 the velarium and from the true velum of the Craspedotae. 



§ 71. Urticating organs (" nematillae," " nematophora," " organa urticantia," n). 

 In all Medusae, as in Acalephae or Cnidaria in general, special organs are formed from 

 the epithelium at definite parts of the body, which are essentially composed of nema- 

 tocysts (" cnidoblasti"), and are therefore termed urticating organs or "nematillae." 

 In the Medusae these are for the most part products of the ectoderm, whilst the endo- 

 derm only forms nematillae in a few places, as for example, on the gastral filaments and 

 in the oral cavity. The urticating organs serve chiefly as weapons of attack and defence 

 (as. for example, the tentacles), but at the same time also as firm supports of the soft body 



