42 



ACALEPIIS IN GENERAL. 



Part I. 



since Quoy ^ and Vogt - avouUI remove tlie Beroids not only from that class, but 

 even from the type of Radiata, and refer them to the lower Mollusks in the 

 vicinity of the Ascidians. It seems hardly credible, that the author of an exten- 

 sive and highly valual)le monograph upon the swinnning Ascidians'^ sliovdd enter- 

 tain such an t)pinion. Every idea of typical plans of structure, as a guide in the 

 general elassincation of the animal kingdom, must be given up Ijy those who would 

 associate animals that are so distinctly radiated as the Ctenophora^ with others in 

 which the liilateral type is so evident as in the Tunicata, and place them in an 

 intermediate i)osition between the latter and the Bryozoa. A general comparison 

 will be sufficient to show that the Ctenophora^ or Beroid Medusae are truly 

 Eadiata. This may best be seen in our Idyia {Flij. 0), where the central mouth, 

 surrounded jjy a circular tube, leads into a vast digestive 

 cavity, above which arise two horizontal tubes, each dividing 

 into four l)ranches. These branches follow the surface of the 

 cylindrical, sligiitly compressed walls of the animal, and unite 

 again with the circular tube encircling the mouth. On the 

 outer surlhce of the body extend eight vertical rows of flap- 

 pers, whose u})pei- ends converge to a central knob at the 

 summit of the animal. The rows of flappers, the hollow tubes, 

 the central mouth, the rosette at the sunnnit, every essential 

 feature in the structure of these animals, is as strictly radi- 

 ated as in any other Radiata in which indications of a, bilateral 

 arrangement are suljonlinate to the general plan of radiation. 

 These subordinate features in the genus Idyia> consist of two additional radiating 

 tubes along the sides of the aniuuil, in the flattening of the digestive cavity which 

 exists also in all the Poly[)s, and in the eccentric position of the doul)le anus. 

 This eccentricity of the terminal end of the alimentary canal occurs, however, in 

 the UKijority of Echinoderms, as well as in the Ctenopliorai, only that in Echi- 

 noderms the anus is simple. But the Ctenophonv are not only radi;ited ; they, 

 in fact, are radiated after the fashion of the other Acalephs, and ought to remain 

 associated with the common Medusa, as they have Ijeen ever since Cuvier distin- 

 guished these animals as a class. 



The special homologies of the Ctenophora" and true Medusa" are most striking. 

 A comparison with Aurelia will at once show this. From the main cavity arise, 



Idyia roseola, Ag. 

 Anal oponitig. — h Later.al radi- 

 ating tube. — c Circular tubt-. — 

 dtfk Vertical lows of locomo- 

 tive fringes. — J? The locomotive 

 fringes sceu in profile. 



^ QuoY et Gaim.vud, Voyage de rAstrulalu', 

 Zoolop;ic-, vol. 4, j). .'JO. 



^ Vogt (C), Zoologisclie Briufc, Frankfurt a. JI., 

 1851, vol. 1, p. 254. 



^ Vogt (C), Ecdicrches siir Ics aniinaux in- 

 ferifurs ik' la Muiliterranee ; 2d. jMrmnirc, siir le.s 

 Tuni<acrs nageaiit.s de la mer dc Nice, ]Meiii. de 

 riiistitut national genevois, Geneve, 1854, vol. 2. 



