90 THE POPULAR SCIENCE MONTHLY. 



other, giving to the odd, phosphorescent, living tubes a lengthwise 

 movement through the ocean. 



The Ascidians a family of fixed Tunicates present societies to 

 some extent organically connected. They are grouped by a common 

 connection of their mantles, or rise successively from a common stem, 

 through which an organic unity is established. Yet their individuality 

 continues ; for, if one of the Ascidians has its circulation cut off, by 

 a ligature, from the common stem, it continues to exist independently. 



In the fixed polyps the subordination of character resembles that 

 of the Ascidians. It is carried further, however. Thus, in some in- 

 stances, not only is the common stem fed by the efforts of a series of 

 individual mouths, but there seems to be a sensitive connection. If, 

 for instance, one of the expanded animals of an Alcyonium community 

 be touched, not only does this animal contract, but gradually the re- 

 maining animals of the community contract also. 



Again, in the Ilydrozoa, individual members of the community are 

 specialized as reproductive organs, being fed through the common 

 stem by the feeding individuals. In these cases the merging of indi- 

 viduality has extended much beyond the simple case of the Salpne, cer- 

 tain members of a society being specialized as organs of a compound 

 animal. These rei^roductive buds, however, in many cases regain their 

 individuality in a very j^eculiar manner. They separate from the com- 

 mon stem, and continue to exist as free-swimming animals. But their 

 specialized development has produced material modifications in their 

 form and internal organization. They are no longer fixed polyps, but 

 free Medusffi, retaining only a general resemblance to the polyp type, 

 and swimming by means of contractions of their umbrella-like disk. 



By this strange modification of the polyp form, to achieve special 

 purposes, a new free animal form is produced, which sometimes follows 

 its new line of development so as to yield an animal markedly distinct 

 from its unspecialized brothers of the same community. Such is one 

 of the many strange modes in which Nature has sought to produce new 

 forms of animal life. 



But the greatest subordination of individuality is shown in the 

 Siphonophorse, a family of Hydrozoa in w^hich a distinct effort seems 

 to be made to attain the elongated, free-swimming form, through com- 

 bination. In some of these the evolution of a colony into a single ani- 

 mal is almost complete. A large number of individuals are connected 

 by a common stem ; but these individuals are so specialized in function 

 as to be no longer capable of a separate existence. They have lost 

 certain powers, and developed others, so that they are reduced to the 

 condition of special organs of a single animal. Some act as food-catch- 

 ing organs, some as mouths, some as reproductive and nursing members, 

 and, by a strange transformation, some have become bell-like organs, 

 which, by successive contractions, expel the water, and force the whole 

 community through the seas. 



