172 



Upon Animal Individuality. By THOMAS H. HuXLEY, 

 F.R.S., R.N.* 



The lecturer first briefly described the structure of the Di- 

 phydoe and Physophoridee — pointing out the general confor- 

 mity of these animals with the common Hydra. 



They differ, however, in this important respect ; that the 

 body in which the eggs are developed is in Hydra, a simple 

 process ; while in the Diphydae and Physophoridse the corre- 

 sponding body presents every degree of complication from 

 this form, to that of a free-swimming, independent " Medusa." 



Still more striking phenomena were shewn to be exhibited 

 by the Salpse. In this genus each species has two forms. 

 In the example chosen these forms were the S. democratica, 

 and the S. mucronata ; the former is solitary, and never pro- 

 duces ova, but develops a peculiar process the " gemmiferous 

 tube ;" upon which, and from which, the associated Salpss 

 mucronatse are formed by budding. 



Each of these carries a single ovum, from which the first 

 form is again developed. 



The Salpa mucronata, which is thus produced from the 

 Salpa democratica, is just as highly organised as the latter. 

 It has as complete a circulatory, nervous, and digestive appa- 

 ratus, and moves about and feeds as actively ; no one unac- 

 quainted with its history would dream of its being other than 

 a distinct individual animal, and for such it has hitherto 

 passed. 



But the Salpa mucronata has exactly the same relation to 

 the S. democratica that the free-medusiform egg-producing 

 body of Physalia or Velella has to the Physalia or Velella ; 

 and this free-medusiform body is homologous with the fixed 

 medusiform body of Diphyes ; which again is homologous 

 with the semi-medusiform, fixed body of a Tubularia, and with 

 the egg-producing process of the Hydra. 



Now as all these bodies are homologous with one another, 

 one of two conclusions is possible ; either, considering the 



* Read at Meeting of Royal Institution, April 20, 1852. 



