OF ARTS AND SCIENCES. 189 



are in reality the lowest, as their tentacles are indefinite in 

 number. Among the Actinoids there are species provided at 

 first with one mouth, which afterwards contracts and divides 

 into two, each being surrounded by its row of tentacles, the 

 animal being thus double above, but single below ; and the 

 division may be repeated, so that the number of mouths 

 shall be four, belonging apparently to as many individuals, 

 while in reality they are but one, being united below. The 

 multiplication is indefinite in many types. Such a pecu- 

 liar structure naturally leads to the question, What defines 

 the individual in this case, — the possession of a single 

 mouth, or the union of all the branches below ? In Meandri- 

 na a number of mouths are surrounded by a single row of 

 tentacles, and there is one common digestive cavity. In a 

 Madrepore, which has sprung from a single egg, the main 

 polyp may preserve its position at the top of the stem, while 

 buds are pushed out from the stem, constituting a community 

 of individuals subordinate to the principal one. This shows 

 distinctly that polyp communities are combining into higher 

 unities. Among some of the Alcyonoids, as in Renilla, Pen- 

 natela, &c., a community of individuals based upon a single 

 stem, each polyp being provided with its own set of tentacles, 

 and all communicating with a common cavity, has the power 

 of changing its position and moving about freely, exhibiting a 

 new kind of individuality, a community moving as a single in- 

 dividual. Among the Polypi, then, the compound individual 

 presents the higher type, and Alcyonoids, which are all com- 

 pound animals, are higher than Actinoids, among which alone 

 simple polyps are found. This position accords with the reve- 

 lations of geology, the former never occurring as fossils in an- 

 cient formations, while the latter have built up all the coral for- 

 mations of past geological epochs. Dana has shown that the 

 Actinoids bud in two ways, some dividing at the top, others 

 budding laterally. Professor Agassiz regards the Madrepores 

 as the highest, on account of their tentacles being definite in 

 number, and some preserving a top animal ; next he places 



