ANATOMY AND PHYSIOLOGY OF SALPA AND PYROSOMA 53 



In the Treiiiatoda there are frequently three " zooid " forms to the 

 individual. 



In the Aphidje the sum of from nine to eleven " zooids " composes 

 the individual, the great number of zooid forms in this case being 

 simply an instance of that " irrelative repetition " of parts so common 

 among the lower animals. 



A similar irrelative repetition exists among the so-called " com- 

 pound " animals, the Polypes and compound Ascidians ; and con- 

 sistently with the present theory we must call a Sertularia or a 

 Pyrosdma, for instance, not an aggregation of individuals into a 

 common mass, but an individual which is developed into a greater 

 or less number of zooid forms, which in the present case remain united. 



Thus the stem and branches of the polypidom in Sertularia are 

 " organs," the ovarian vesicles are " organs,'' the polypes are '' zooids ; " 

 the sum of the organs and zooids constitutes the individual. 



If the separate polypes be individuals, what is the polypidom which 

 exists before them, and therefore cannot be derived from them ? 



It seems startling to assert that a Salpa of the form A with some 

 fifty or sixty of the form B which proceed from it, constitutes but one 

 zoological individual ; still more to aver this of some millions of 

 Aphides all proceeding indirectly from one ovum ; but these diffi- 

 culties have reference merely to our ordinary notion of individuality, 

 and involve us in no self-contradictions and inconsistencies such as 

 seem inherent in any other view of the case. 



Section II. — The Anatomy of Fyrosoma. 



41. This genus, first established and very imperfectly described by 

 Peron,^ received elaborate investigation from Lesueur" and from 

 Savigny,^ who very carefully described every part of its organiza- 

 tion with the exception of the generative organs, and one or two 

 other points of minor importance. 



Subsequently M. Milne-Edwards* showed that the nature of the cir- 

 culation was the same in it as in the other Ascidians. 



Of the three species distinguished by Lesueur the present appears 

 most closely to resemble the P. atlanticiim. 



42. The only specimen of this remarkable animal which the writer 

 has had an opportunity of examining in the fresh state, was procured 

 ■on the night of the 15th of June 1850, in about 4S"'8s S. lat. and 



I io''30 W. long. The sky was clear but moonless, and the sea calm ; 



' Annales du Museum, 1804. '^ Journal de Physique, 1815. 



' Mem. sur les Animaux sans Vertebres. ' Comptes Rendus, 1840. 



