Jaekd on Orthoceras.—Riiedcinann. 205 
ous. For the fixation at one point allows a balancing move- 
ment in all directions and may be conceived to have been af- 
fected by vertically stretched ligaments, which, ring-like, em- 
brace the initial shell. The orthogenetic* formation of the 
chambers exerts an increasing pull upon the chamber of at- 
tachment, and as a completed structure, by the law of accelera- 
tion, appears in successively earlier stages of the ontogeny, it 
is obvious that cameration finally led to detachment among a 
part of the forms. 
"e) The siphonal cord which has always been the most 
peculiar enigma in the cephalopod organization, has found 
very different interpretations, but none of these has been ac- 
cepted as wholly satisfactory. It can hardl}^ be considered as 
an organ of fixation for the shell, because, on one hand the 
animal is held in the living chamber by a muscle of attach- 
ment, and oil the other hand the isiphuncle in Nautilus has by 
no means the histologic character of a ligament, and finally it 
would remain unintelligible why it then perforates the whole 
shell throughout lifetime and is not concentrated upon the last 
septal wall. 
"If v/e now proceed from the idea that the chambered cep- 
halopod shell originated from a sessile protoconch, the forma- 
tion of the siphuncle appears at once in quite difl:"erent light. The 
sipJmnclc is then nothing hut a portion of the body restricted 
by the formation of the chambers. It becomes biologically 
analogous to the umbilical cord of the nmuimaJs, more specially 
hoivever to that portion of the pelmatozoans, u'liich is con- 
stricted by the stem joints, and these analogies appear also for 
that reason to be not without meaning, for various facts point 
to phylogenetic relations of these types. I will hereby exclude 
at once the supposition that I would derive the vertebrates 
from the cephalopods or pelmatozoans, on the contrary, in my 
opinion, some facts seem to argue for a derivation of the 
mollusks as well as of the echinoderms from higher types of 
the group of the episomatides by repression of development 
(O. Jaekel. Uber die Stammform der Wirbeltiere ; Sitz.-Ber. 
Ges. naturforsch. Freunde, Berlin, 1896, S. 116). 
"According to this view the siphonal cord would enclose 
a portion of the somatic cavity, which kept the embryonic body 
* O. JAEKBL. Ueber verschiedene Wege pbvlogenetischer Entwicklung. 
Gust. Fischer. Jena. 1902. 
