` 
1885.] On the Larval Forms of Spirorbis borealis. 247 
Ichthyopterygia, derived their being from them. The phylogeny 
may be thus expressed : 
Dinosauria Testudinata Rhynchocephalia Lacertilia Ophidia 
(Crocodilia) | / 
. W S / 
Pterosauria n Pythonomorpha 
<A Ss 
~~ Dinosauria Sauropterygia ~ 
S 
Ichthyopterygia Theromorpha 
(To be continued.) 
:0: 
ON THE LARVAL FORMS OF SPIRORBIS BOREALIS 
DAUDIN. 
BY J. WALTER FEWKES. ; 
NT URALISIS who are engaged in the identification of the 
larval forms which marine animals pass through in their 
growth from the egg, find great difficulty in this study from the 
lack of direct observations in raising these larvæ from the eggs 
or in rearing them directly into the adult. This is particularly 
true in regard to the young of marine annelids, a most profitable 
field for new observations and one which has had but few culti- 
vators among American naturalists. The following paper is 
offered as a help to those engaged in this study and not as an ex- 
tended account of the embryology of the animal of which it 
treats. Itis especially intended for those who are interested in 
the identification of our marine annelid larvz." 
A genus of chztopod annelids called Spirorbis, as is well 
known, in its adult and older larval stages, secretes a coiled cal- 
=- Careous case, commonly called its, shell, in which it lives. This 
case is permanently cemented or attached to some foreign body, 
from which fact the adult is incapable of locomotion. Not so, 
however, the larva, which is destitute of any such shell, is not 
fixed but is free swimming, and often captured with the dip-net in 
surface fishing. From the great dissimilarity in outward form as 
1 The observations here recorded were made in the Zodlogical Laboratory at New- 
port, R. I, I am indebted to Mr. A. Agassiz for facilities to carry on my studies at 
that place. 
