124 ACALEPHS IN GENERAL. Part I. 



the Hydroids resemble the rol\ps, -vvitli Avhioh, indeed, they have been united as 

 members of the same class ; while the Discophorai jiroper constitute the character- 

 istic group of Acalejihs, the group which has always been considered the typical 

 group of this class. The Ctenophortc bear the same relation to Echinoderms as 

 the Hydroids bear to the Polyps; and this resemblance of the Ctenophoraj and Echi- 

 nodei'ms is especially recognizable in the peculiarity of their vertical chymiferous 

 tubes Avith their locomotive flappers, and the homology there is between them and 

 the ambulacra] system of the Echinoderms. But neither the resemblance of Hy- 

 droids to Polyps nor that of Ctenophorre to Echinoderms is a real indication of 

 affinity : it is only an analogy, arising from a similarity of form in parts which 

 have only a general homology, and no special homology with one another. But 

 this analogy, once recognized, has its significance. It confirms the views presented 

 above respecting the relative standing of the three orders of Acalephs. Hydroids, 

 as the lower order of Acalejdis, are analogous to the Polyps, the lowest class of 

 Radiates ; Discopliora^ the most characteristic type of Acalephs, occujjy a middle 

 position between them and the Polyps, as the Acalephs, considered as a class, 

 occupy an intermediate position between the Polyps and Echinoderms ; and the 

 Ctenophorte, as the highest order in the class of Acalephs, correspond to the Echi- 

 noderms, and especially to the Holothurioids, the highest oi'der of the highest class 

 among Radiates. 



Such analogies may bo traced in other classes of the animal kingdom. As- 

 suming that the Articulates emln-ace only three classes, — the Worms, Crustacea, 

 and Insects ; and that the Insects themselves form only three orders, — Mj-riapods, 

 Arachnids, and Insects proper, no one can fail to perceive the analogy between 

 the Myriapods as the lowest order of Insects, and the Worms as the lowest class 

 of Articulates, or between the Arachnids as the second order of Insects, and the 

 Crustacea as the second class of Articulates ; and the highest order among Insects 

 consists of those best representing the character of the class of Insects, which 

 stands highest among Articulates. Perhaps objections may be raised against this 

 primary division of the Insects into three orders, and perhaps also against the 

 division of Articulates into three classes ; but to my mind these analogies Avould 

 have great weight in establishing this classification as correct. Whatever may be 

 said of the analogies alluded to between the orders of Acalephs and the classes 

 of Radiates, I have no hesitation in aflinning that there are only three orders in 

 the class of Acalephs, and that these orders stand to one another in the position 

 I have assigned to them, — the Hydroids ))eing the lowest, the Discophora? next, 

 and the Ctenophorce highest. 



