378 HOMOLOGIES OF THE RADIATA. Part V. 



beyond the main cavity, it is inverted into it, the outer surface assuming digestive 

 functions. We may compare this part to the neck of a bottle, wliich in Polyps 

 would be turned inside, while in Acalephs it is turned out and divided into a 

 number of distinct lobes. In this connection, it is essential to notice that the 

 genital apparatus, extending in Polyps along the free edge of the radiating par- 

 titions, is double; so that there is nothing extraordinary in the position of these 

 organs along the radiating chymiferous tubes of the Acalephs. This does not 

 indicate a different position, but is the result of the great thickness and width of 

 the interambulacral zones of these animals, in consequence of which the genital 

 organs are divided into two rows, one on each side of an interambulacrum, while 

 they appear to be in pairs on each side of an ambulacrum. 



The class characters of the Acalephs are as distinct as their homologies Avith 

 the other classes of Radiates are intimate. The bulk of the body is a continuous 

 mass, traversed by narrow tubes arising from a central cavity, the opening of 

 which forms a more or less prominent proboscis. Even the most Polyp-like Hy- 

 droids have no trace of radiating partitions. The central cavity corresponds to 

 the main cavity of the body of Polyps, and the radiating tubes to the radiating 

 chambers. As in Polyps, the primary tentacles are in the direct peripheric pro- 

 longation of the ambulacral system; but, in consequence of the great development 

 of the interambulacra, the genital organs are more differentiated, and often assume 

 an extraordinary development, in connection with a system of special interambu- 

 lacral radiating tubes, as exist, also, in some Echinoderms. The periphery of the 

 ambulacral system becomes connected, either by a marginal circular tube, or by 

 a network of anastomoses, which are also to be found in many Echinoderms. 

 The proboscis, when it assumes the shape of a tube, and the so-called arms, around 

 the mouth, which are only a special mode of development of the proboscis, are 

 homologous to the inverted neck of the Polyps, suspended in their main cavity. As 

 the special homologies of the different orders of Acalephs have already been dis- 

 cussed in this volume, I need only say here, that, whether the members of this 

 class are as simple as the Hydroids and naked-eyed Medusae, or as complicated as 

 the highest Discophora? and Ctenophoraa, the same homologies may be traced among 

 them all, with corresponding class differences. It does not matter,, for instance? 

 whether the radiating tubes are simple or branching; whether their course is limited 

 to the ambulacra or extends to the interambulacral zones; Avhether they trend in 

 the same plane, or branch up and do\vn in the direction of the actinal and abac- 

 tinal areas ; whether tentacles exist only in the prolongation of the ambulacral tubes, 

 or are also scattered along the circular tube ; even their presence or absence, and 

 the presence or absence of eyes upon or between them, are of subordinate im- 

 portance, as are also the preponderance of the actinal over the abactinal area, and 



