HYDROZOA, SCYPHOZOA, ACTINOZOA, VERMES WHITELEGGE. 381 



The foregoing table, although not exhaustive, exhibits a wide 

 difference between the two species, especially in the number of 

 ventral siphons and the secondary tentacles of the basal cor- 

 midia. 



In Physalia megalista the lowest number of siphons is 35, the 

 highest 78 ; in P. utriculus the lowest is 136, the highest 302, or, 

 leaving out the large specimen, 164. The secondary basal tentacles 

 in the former vary from 1 to 4, and in the latter from 6 to 22. 



There are other important characters, which exhibit a number 

 of differences in the length, colour, or distance of one part from 

 another ; some of these, although varying slightly in themselves 

 within certain limits, are pretty constant in each species, and are 

 very evident when the two species are compared. They may be 

 enumerated as follows : 



P. megalista. 



Crest of pneumatophore long 



Apical crestless portion short 



Distance between ventral and basal 



cormidia short 



Length occupied by basal group of 



cormidia long 



Apex of pneumatophore green 



Summit of crest magenta 



Mouths of siphons yellow 



P. utriculus. 



Crest of pneumatophore short 



Apical crestless portion long 



Distance between ventral and basal 



cormidia long 



Length occupied by basal group of 



cormidia short 



Apex of pneumatophore blue 



Summit of crest Campanula blue 



Mouths of siphons white 



With a view of testing the pneumatophore to see if it would 

 yield any reliable specific character, I have carefully measured a 

 series of living, dead, and preserved specimens. I am well aware 

 that the pneumatophore is a very variable structure ; but, as in 

 most other organisms, when at ease or in a restful condition, it 

 has a certain definite form which may be regarded as the shape 

 of the living object when in a healthy normal state. In the follow- 

 ing measurements as far as the material would allow specimens 

 have been selected that came nearest to what I regard as the 

 natural shape of the pneumatophore. 



