I 



THE CTENOPHORA 13 



The characters of the Ctenophora which are relied on as 

 evidence of their Coelenterate nature are as follows: — 1. The 

 existence of a gastrovascular system, and the absence of a separate 

 body cavity or coelom. 2. The general shape and architecture of 

 the body, its radial symmetry, and the existence of an abundant 

 gelatinous material between the two primary layers — the ecto- 

 derm and endoderm. 3. The presence of tentacles, which are 

 likened to those of a Medusa. 4. The position of the gonads, and 

 the derivation of the sexual cells from the endoderm. 5. The exist- 

 ence of a sub-epithelial nerve plexus resembling that of Medusae. 



6. The supposed homology between lasso-cells and nematocysts. 



7. The absence of nephridia. In a more special manner it has 

 been sought to compare the Ctenophore directly with a Medusa 

 or with an Anthozoan zooid. Thus the general surface of the 

 Ctenophoran body has been homologised with the exumbrellar 

 surface of a Medusa ; the stomodaeum with the sub-umbrellar cavity ; 

 the gelatinous mesoderm of the one with the mesogloea of the other; 

 the gastrovascular canals Avith the radial canals ; the Ctenophoran 

 tentacles with the marginal tentacles of the Medusa. These homo- 

 logies appeared at one time to be established beyond all cavil by 

 the discovery of Ctenaria ctenophora, a Cladonemid Anthomedusa, 

 described by Haeckel (12) as a form directly intermediate between 

 the Hydromedusae and the Ctenophora.^ Ctenaria (see Fig. V.) 

 is an ovoid Anthomedusa, with a relatively small sub-umbrellar 

 cavity, the aperture of which is still further diminished by the velum. 

 The mouth opens at the end of a manubrium, and is surrounded by 

 a circlet of sixteen oral tentacles. The gastral cavity is divided 

 by a constriction into an upper and a lower moiety, the former 

 of which is homologised with the infundibulum of Ctenophora. 

 From the lower moiety four perradial gastrovascular canals are 

 given off, each of which bifurcates to form two adradial canals. The 

 eight adradial canals thus formed are connected round the margin 

 of the umbrella by a riiig canal. There are two perradial marginal 

 filamentous tentacles beset with accessory filaments. At the base 

 of each tentacle is a pocket-like cavity in the exumbrella, lined by 

 batteries of nematocysts ; it is doubtful whether the tentacles are 

 retractile within these pouches. On the surface of the exumbrella 

 are eight adradial meridional ridges, made up of nematocyst 

 batteries. There is no apical sense organ, and the gonads are 

 borne, as in all Anthomedusae, on the maiuibrium. The 

 resemblance of Ctenaria to the Ctenophora is quite superficial. 

 One has only to compare the eight nematocyst stripes of the one 

 with the highly specialised ciliated costae of the other to see their 



^ "Eine neue hochst interessaute pacifische Form, Ctenaria ctawphora, welche 

 ich als eine unmittdhare Uebcrgangsform von Gemniaria-iilinliclien Anthomedusen 

 zu Cydippe-iihnlichen Ctenophoren auffassen muss." 



