12 CTENOPHORvE. 



from the rest of the digestive apparatus, exactly corresponding to what 

 exists in Echinoderm larvae. The connection between the water system 

 and the digestive system is likewise precisely similar to that of Echino- 

 derms in their larval state ; for although in the adult Star-fish, or Sea- 

 urchin, or Ophiuran, there is no apparent connection between the am- 

 bulacra! and the digestive system, yet in the young larvae we can see 

 that this connection exists, the water system being formed by diver- 

 ticula from the digestive cavity ; while the injections of Professor Agas- 

 siz have proved the existence, in the adult, of a similar connection in 

 Echinarachnius, in Mellita, and in Clypeaster. 



It was only after the embryos of Echinoderms had been compared 

 with Ctenophoroe that undoubted evidence of their identity of plan was 

 obtained. The embryological development of Ctenophoroa leaves no 

 doubt as to the Acalephian character of the order. It remains only for 

 us to see whether the Ctenophora3 form a group of equal value with the 

 rest of the Acalephs, or stand simply as an equivalent of the other two 

 orders, the Discophorae and the Hydroids. The careful examination 

 lately made of many genera of which we had no definite knowledge 

 before, as well as their embryology, has now left it difficult to decide 

 whether the Discophorae and Hydroids are independent orders, or wheth- 

 er the distinction established between the Discophorne and Hydroids is 

 merely a subordinal division in a great order, including these two. If 

 so, this order might be called the Medusido?, in opposition to the Cteno- 

 phoro?, which are an order perfectly and accurately circumscribed ; the 

 presence of locomotive flappers being as characteristic for the Cteno- 

 phorne, and as constant a feature of Ctenophorrc among Acalephoe, as 

 feathers are for the class of Birds among Vertebrates. These flappers 

 exist almost from the earliest embryonic stages, and thus far not a 

 single exception is known to the rule. Fritz Miiller and Agassiz have 

 shown that it is hardly natural to associate the Charibdeidce and JEg'i- 

 nidoe with the Hydroids, and the latter has proposed to unite them with 

 DiscophonB, while the former would make a separate order of them. 

 This seems hardly justifiable, as there are as many reasons their mar- 

 ginal appendages, genital organs, c. for uniting them with Disco- 

 phorae, as for leaving them with the Hydroids, the shape of the bell, 

 the great development of the veil. If, in addition, we take into account 



what we have observed in the Trachvnemidoe, it will be seen that we 



/ 



can no longer draw the line between the Discophorae and Hydroids as 

 distinctly as before ; while the creation of a third group equivalent 

 to these two, to contain the families in dispute, does not bring us any 

 nearer to the solution of the problem. A more accurate knowledge of 

 the tropical forms will go far to settle this point ; and in the mean 

 while, with this explanation, I will place temporarily (until further 

 information can be gained) the ^Eginida) and the Trachynemidse among 



