METAMORPHOSES OF ANIMALS. 107 



recognize only three classes, Polypi, Acalephce, and Eclii- 

 nodermata. The chief difference between the two first 

 lies in the radiating partitions of the main cavity of the 

 Polypi, supporting the reproductive organs ; moreover the 

 digestive cavity in this class consists of an inward fold of 

 the upper aperture of the common sac of the body, while 

 in Acalephs there exist radiating tubes, at least in the 

 proles medusina, which extend to the margin of the body 

 where they anastonioze, and the digestive cavity is hol- 

 lowed out of the gelatinous mass. This is equally true of 

 the Hydroids, the Medusae proper, and the Ctenophorae ; 

 but nothing of the kind is observed among Polypi. Si- 

 phonophorae, whether their proles medusina becomes free 

 or not, and Hydroids, agree in having, in the proles me- 

 dusina, simple radiating tubes, uniting into a single cir- 

 cular tube around the margin of the bell-shaped disk. 

 These two groups constitute together one natural order, 

 in contradistinction to the Covered-eyed Medusae, whose 

 radiating tubes ramify towards the margin and form a 

 complicated net of anastomoses. Morphologically, the 

 proles polypoidea of the Acalephs is as completely an 

 Acaleph as their proles medusina? and, whether they 

 separate or remain connected, their structural relations 

 are everywhere the same. A comparison of Hydractinia, 

 which is the most common and the most polymorphous 

 Hydroid, with our common Portuguese Man-of-War, (Pliy- 

 salia,) will at once show the homology of their most poly- 

 morphous individuals. 2 



The embryology of Mollusks has been very extensively 



1 I shall show this fully in the 2 It has already been stated above, 



third volume of my contributions to that the Milleporina are not true 



the Natural History of the United Polyps, but Hydroids, closely allied 



States. Meanwhile, see niy paper on to Hydractinia, by their structure 



the structure and horuoloyies of lla- and their polymorphism, 

 diata, q. a., p. 28. 



