NOTES ON THE EMBRYOLOGY OF LIMCTLUS. 
535 
have the nuclei placed at the ends away from the lumen. 
Fiom this point the tube bends back on itself, going back a 
short distance, and then turning again enters the fourth 
segment, where it turns again and comes back to its first turn, 
where it terminates. As to the character of the termination I 
am yet in doubt, although I have examined many sections, 
both transverse and longitudinal. In some it appears to com- 
municate directly with the body cavity, the internal end being 
open (fig. 27 ,/«.). I have been unable to detect cilia in any part. 
In various parts of the tube the epithelium varies in character 
between columnar and quadrate cells. In all the cells the 
nuclei are very large, and are placed at times nearer the fore, at 
others to the deeper end of the cells. The general character of 
the quadrate cells is shown in fig. 28, which closely resembles 
Professor Lankester’s delineations of these glands in the adult 
Limulus. 
I see no reason why these glands in Limulus and the corre- 
sponding ones in the Scorpion, together with the so-called 
shell-glands of the Crustacea, should not be regarded as seg- 
mental organs. Later in this paper I will return to their 
discussion, but here I would call attention to the fact that at 
least in the later stages the inner end of the gland terminates 
csecally as in the various Crustaceans. The closure of its 
efferent duct takes place later. 
Respiratory Organs. 
On a previous page I have described the early stages of the 
abdominal appendages. They arise as broad lamellar out- 
growths from the lower surface of the abdomen. At first, and 
in fact until the appearance of the gill books but two of these 
appendages are visible. These correspond to the operculum 
and first gill-bearing appendage of the adult. The others arise 
in regular sequence until the whole number (five) is reached. 
At first each of these appendages is simple, nothing that could 
be interpreted as a biramous condition appearing until the 
stage of fig. 12. One fact requires mention here : these appen- 
dages are from the beginning broad and leaflike, differing in 
