14 
Siphonophora rosarum 
Summary ( cont.). 
The Apterous Parthenogenetic Viviparous 
Female. 
Nervous System. 
Great concentration of the ganglia, 
especially of the thoracic ganglia. 
Sense Organs. 
The sixth joint of the antenna is covered 
with curious markings. 
Compound eyes (about 60 facets) with a 
small supplementary eye. 
Tracheal System. 
A complicated dorsal and ventral system 
present formed by the division of the main 
trachea from each stigma into dorsal and 
ventral branches which join up to form the 
respective systems. 
The Winged Parthenogenetic Viviparous 
Female. 
Nervous System. 
Concentration more marked than in the 
apterous form and also a relative increase 
in the size of the ganglia. 
The wing muscles are apparently only 
supplied by the nerves from the second 
thoracic ganglia. 
Sense Organs. 
Ridges are present on the sixth joint of 
the antenna and the third joint is covered 
with a number of tubercles. 
A few tubercles are also present on the 
fifth and sixth joints. 
Eyes larger than in the apterous form 
(about 80 facets) and the supplementary eye 
is much more prominent. 
Tracheal System. 
No details of the systems obtainable, but 
the main trachea from each stigma divides 
in exactly the same way as in the apterous 
form. 
The foregoing comparison of the anatomy of the apterous vivi¬ 
parous and the winged viviparous stages of S. rosarum although not 
revealing any very great differences, may be regarded as supporting the 
view that the apterous stage is a degeneration from the winged stage. 
The fact that so much similarity exists between the two forms is 
perhaps more easily explained by supposing that the habit of flight 
had become eliminated, possibly through disuse, resulting in the 
consequent atrophy and disappearance of the organs directly con¬ 
cerned with that function; for if the other view be considered, it 
would be reasonable to suppose that more specially marked differences 
in an upward direction would be the result of such a vital change 
in the habit of the creature. There is one point of difficulty, how¬ 
ever, and that is to account for the difference in form of the alimentary 
canal, but, as has been already suggested, this may be due to other, 
quite different, and at present unknown, causes. 
