Chap. II. THE CLASSES OF RADIATA. 67 



the uppei" part of the radiating cavity formed by the l3ody--\valls arise hxterally 

 more or less uumerous hollow tentacles, which are also in direct and free com- 

 munication with the radiating chambers. In fact, the tentacles are simply lateral 

 diverticles of the upper part of the chambers. The centre of the upper part of 

 the sac is widely open, but that opening, generally called the mouth, is not the 

 open edge of the sac ; it is the result of the inversion of the upper central part 

 of the body-wall, the outer surface of which, in consequence of this bending inward, 

 becomes internal, and forms what is commonly called the stomach. An accurate 

 idea of this structure may be fonned by comparing the sac of a Polyp to a bottle, 

 the neck of which should be turned outside in, and expanded into another sac 

 concentric to the l^ody. This pendent sac, or stomach, is open at the bottom, and 

 this opening leads into the main cavity of the 1jody. The lower opening of the 

 digestive cavity is, therefore, properly speaking, the outer opening of the body-wall, 

 and strictly homologous to the mouth of the Acalephs. The habit of Actinia?, of 

 turning this so-called stomach inside out, affords an excellent opportunity to trace 

 this homology, when it becomes plain that the opening commonly called mouth 

 in Polyps in no way corresponds to the luouth of the Acalephs. It is equally 

 plain, from such a comparison, that the so-called stomach of the Polyps is not any 

 more homologous to the so-called stomach of the Acalephs. This stomach of the 

 Acalephs can only be homologized with the open space in the centre of the main 

 cavity of the Polyps, with which the radiating chambers stand in the same re- 

 lation and communication as the radiating tuljes of the Acalephs to their so-called 

 stomach. The fluid circulating through the so-called gastero-vascular system of the 

 Acalephs is chyme, and nothing but chyme mixed with watei', as I have shown 

 in my contributions to the Natural History of the Acalephs of North America, 

 Part I. page 2G3. 



These facts are in themselves sufficient to distinguish the Polyps under all 

 circumstances, not only from the higher Acalephs, but also from the Hydroids, in 

 which the structure is as essentially Acalephian as in the Medusae proper. For 

 many years past I have insisted upon these differences, and I truly wonder tliat 

 there are still naturalists Avho do not see how completely distinct the structural 

 type of the Polyps is from that of the Acalephs. In my lectures on Comparative 

 Embryology,^ delivered in the winter of 1848 and 1849, I have already shomi, not 

 only how the Hydroids differ from the Polyps, Init also how the Hydroids agree 



1 Twelve Lectures on Comparative Embryology, the Structure and Homologies of Radiated Animals, 



delivered before the Lowell Listitute, in Boston, in witli Reference to the Systematic Position of the 



December and January, 1848-1849, Boston, 1849, Hydroid Polyps," Proc. Amer. Ass. of Sc. Cam- 



8vo. fig. pp. 42 and 43. See also my paper "On bridge, 1849, p. 389. 



