114 ACALEPIIS IN GENERAL. Part I. 



which the natuml orders have Ijeeu most fully investigated. It is so among Poljjjs, 

 if the Actinoids and Halcyonoids constitute natural orders in that class ; for the 

 Halcyonoids, with their eight spheromeres, and lohed tentacles, stand higher than 

 the Actinoids. It is so among Echinoderms, the orders of which truly correspond 

 to diflercnt degrees of complication of their structure, and most naturally mark 

 the relative rank of these animals. It is so among Crustacea, taking the Eotifera, 

 the Entoniostraca, the Isopods, the Amphipods, the Stomatojjods, and the Decapods 

 as their natural orders. It is so among Acephala, if the Br}ozoa, the Brachiopods,^ 

 the Tunicata, and Lamellibrancliiata constitute natural orders. This gradation, in 

 accordance with the complication of structure, is equally apjjarent among the 

 Batrachians and the true Eeptiles ; and if it is not traceable at present with the 

 same certainty in all the classes of the animal kingdoiii, I am inclined to believe 

 that it is not because this principle is incorrect, but because w'e have not yet 

 obtained a satisfactory standard, by which to determine the relative importance 

 of their structural differences. At all events, a majority of the classes, and those 

 best known to me, coincide with the view I have expressed respecting the meaning 

 of orders. It would ]je surprising should there be some classes in which no such 

 gradation exists, when it is so apparent in others. Let us now see what are the 

 different degrees of complication of structure observed among Acalephs. 



After tracing the special homologies of the Ctenophora?, and ascertaining their 

 close relationship to the ordinary Medusa3, it is evident that they belong to the 

 class of Acalephs; but in this class they constitute a natural and distinct order. 

 Their chief difference from the Discophora) consists in the mode of ramification 

 of the chymiferous tubes, originating in two main trunks, in opposite directions, 

 each of which is divided into two horizontal Ijranches, and each branch into two 

 horizontal forks ; so that the number of horizontal chjiniferous tul)es is always 

 eight. But, unlike other Acalephs, these tubes do not terminate at the periphery, 

 but open into eight vertical branches, converging in opposite directions towards 

 the actinal and the abactinal ends of the body, and giving out minor branches 

 into the spherosome. The main trunks of these vertical branches are parallel to 

 the surface of the spherosome, and follow the same course as the rows of locomotive 

 flappers, which extend, like eight ril)s, upon the surfiice of the body. Towards the 

 actinal and towards the aljactinal poles of the spherosome, the vertical branches of 



^ Tlir position I have assigned to the Brachio- irresistible that Bryozoa and Brachiopods are more 



pods, near the Bryozoa, has been confirmed Iiy a closely related to one another than any other groups 



paper just published, in which a Brachiopod is of Acephala. See Beschreibung einer Brachiopoden- 



describcd, resembling so closely a young Bryozoan larve von Fritz MUller in Desterro (Brasilien), 



just hatched from the egg, that the conclusion is in Archiv fiir Auat. Phys. und wiss. Med. 18G0, p. 72. 



