ON ACEPHALA. 417 
eggs. M. Chamisso discovers some analogy between this 
singular disposition of the salpz, and the metamorphoses of 
batracian reptiles and insects. What seems to confirm this 
opinion is, that there are extremely important differences be- 
tween the two forms of the same species of salpa, not only 
in external conformation, but also in the disposition of the 
muscles, and the position of the viscera. 
In the solitary, and consequently multiparous individuals, 
the body presents none of the appendages, or protuberances 
which are proper to produce a junction. ‘The orifices are 
terminal, the first bilabiate, with unequal lips, and the second 
truncated. In the aggregated or uniparous races, there are in 
different parts of the body appendages, protuberances, or 
spines, by the assistance of which the individuals are linked 
together in a determined order. All the individuals of one 
and the same group are perfectly similar in bulk and length, 
though they may differ much in these respects, in different 
groups. 
The mode of life in the ascidiz, very much resembles that 
of oysters, to which they approximate also in internal organi- 
zation. Most of the species have the habit of grouping to- 
gether in numbers on the same body, where they remain fixed 
all their lives. Sometimes the young are fixed and grow on 
the body of the mother. 
The ascidiz furnish an abundant nutriment to fish, and 
even man feeds on several species. It is thought that they 
place themselves, in preference, at a certain elevation on the 
coasts, to escape becoming the prey of the former. Their 
only mode of defence consists in shooting out the water con- 
tained in their sac, through its two apertures, which is done 
whenever they are touched. Some of them can squirt this 
fluid to the height of three feet. 
The little animals called BOTRYLLI, were for a long time 
very imperfectly known. They were considered as belonging 
VOL. XII. Ee 
