104 ACALEPIIS IN GENERAL. Part I. 



If, then, classes are characterized hy the mode of execution of a given plan, 

 the Ctenophoraj being radiated animals, -\ve have only one jioint more to ascertain 

 respecting them. Does their structure exhibit only a general homology to that 

 of the Acalephs, or are the Ctenophonc linked to the ordinary Acalephs Ijy special 

 homologies? What has already been said when considering their typical relations 

 seems to me conclusive in that respect. Ctenophoiu^ differ only in degree, and 

 not in Idnd, from the animals thus fiir generally considered as true Medusa; they 

 must, therefore, be considered as belonging to the class of Acalephs, in which, as 

 we shall see in the next section, they constitute a natural order. 



As the Discophorne have always been considered as the typical group of the 

 class of Acalephs, and as the Acalephian character of all the other groups that 

 have successively been associated with them, or removed from them, has uniformly 

 been measured by the degree of their affinity to the Discophorce, as soon as it 

 is ascertained that these animals exhiljit a special mode of execution of the plan 

 of radiation, the independence of the Acalephs, as a class, is also proved. And 

 this has already been done in a preceding section (p. Co). In the next, Ave shall 

 consider the position of the Discophori\3 in their class, amidst all the other repre- 

 sentatives of that class. 



The evidence that the Hydroids should be associated in one and the same 

 class with the Discophora? and Ctenophorai is of two kinds. In the first i)lace, 

 Hydroids produce Medusa? ; next, they are not themselves Polyps, as was long 

 admitted. The first of these facts furnishes a direct argument for the necessity 

 of uniting that kind of Hydroids with the other Acalephs; and the circumstance 

 that the Hydroids from which free Medusa? arise are not identical in their structure 

 with Polyps, but themselves reseml)le Medusa? more than Polyps, in connection 

 with what is already- known of the reproduction of the latter, shoAvs that Polyps 

 never produce free Medusa?, but that the Polyp-like animals, from Avhich free Me- 

 dusae arise, are themselves Acalephs. 



It is hardly necessar}', noAvadajs, to demonstrate that such animals as Sarsia, 

 Lizzia, Zanclea, Cladonema, Ilippocrene, Nemopsis, Hybocodon, Tiaropsis, Thaumantias, 

 etc., are genuine Medusa?. Their close affinity to the highest representatives of 

 this order of Acalephs has been recognized Ijy all the investigators of this class 

 of animals. Peron and LeSueur, Eschscholtz, deBlainville, Milne-EdAvards, Lesson, 

 Sars, Forbes, Dujardin, Leuckart, Gegenbaur, and others have expressed their con- 

 viction that they are such, not only ))y direct declarations, but also in various 

 other Avays, Avhen alluding to them. To the arguments adduced by other investi- 

 gators, ncAv facts have been more recently added: their mode of reproduction has 

 been made knoAvn ; their sexual organs have Ijeen studied ; the development of 

 their eggs has been traced through every stage of groAvth ; the formation of their 



