1885.] On the Larval Forms of Spirorbis borealis. 255 
does not immediately settle to the bottom prior to the se- 
cretion of the case in which it lives, but passes through pre- 
liminary stages while floating on the water. Upon the sur- 
face of my aquaria, with its Spirorbis, I found a multitude of 
small white bodies, unattached, which on close examination were 
found to be Spirorbis larve in which the shell had just begun to 
be secreted. They float on the surface for a short time until the 
increasing specific gravity of their bodies sinks them to their 
future homes. 
Fig. 15 represents an example of a larva of this kind in which 
the head and collar is half protruded outside of the cavity of the 
case in which the larva is found. The head and branchial appen- 
dages occupy the middle of the figure at the top, while the ex- 
panded trumpet-shaped structure below it is the half protruded 
collar. It is extremely difficult to draw accurately the outlines of 
a specimen of Spirorbis in this stage when wholly or partially 
expanded and alive, from the fact that the movements are so 
quick in retracting itself into the case, and the animal is/so sensi- 
tive to any small motion in the immediate vicinity. It has been 
almost impossible for me to observe the expanded Spirorbis long 
enough to draw anything more than a simple outline with the 
camera, This difficulty increased with the growth in age of 
Spirorbis. 
The case or shell of the larva, Fig. 15, is not at first coiled, but ` 
slightly curved, horn-shaped, well formed at its larger end, with 
less solid walls at the smaller extremity. The most prominent 
structure in the body of the larva is an oblong mass of cells of 
brick red color seen through the transparent walls of the shell. 
In the next stage (F ig. 16) which was also found floating on the 
surface of the water, the shell has elongated and become par- 
tially coiled, but is still transparent and in places more or less 
- flexible in character. The larva now occupies not more than 
one-half of the whole length of the case when the appendages to 
the head are expanded. Fig. 16 represents this larva taken on 
the surface of the water prior to attaching itself to some foreign 
object. The branchial appendages to the head are more com- 
pletely developed in this than in previous conditions in the 
_ growth of Spirorbis. The ciliation upon them is also more con- 
spicuous than in previous embryos. The operculum is repre- 
sented on the right hand side of the figure. 
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