534 
J. S. KINGSLEY. 
condition of the walls is found in all the embryonic stages, and 
the heart does not have epithelial and muscular layers until 
after hatching. This mode of the formation of the heart is 
paralleled in both Crustaceans and Spiders, and hence throws 
no light upon the affinities of Limulus. 
At about the same time that the dorsal mesoblast begins to 
thicken for the heart a similar thickening is noticeable in the 
epiblast immediately above it (fig. 31). Reichenbach ( ! 77 ) 
has noticed a similar thickening in the Decapods. Of the 
meaning I am uncertain ; it may be that it is the remains of a 
degenerate “ dorsal organ,” but it seems more probable (at 
least in the case of Limulus) that it is the early stage of the 
median dorsal crest or ridge of the adult. In the latest stages 
which I have studied I have found no further change in it. 
Segmental Organs (nephridia). 
Under this head I would place the brick-red glands first 
noticed by Dr. Packard (’ 75 a ), and recently described in detail 
by Professor Lankester (’ 84 ). Concerning their earliest stages 
I am yet in doubt. The earliest trace of them which I have 
seen was in the shape of two patches of mesoblast, one on 
either side in the fifth segment of the body. With growth 
they increase in size, forming a well-defined tube, and join the 
epiblast by the posterior extremity. This junction takes place 
in the posterior coxo-sterual articulation of the fifth pair of 
legs, and soon after an opening appears enabling the organ to 
communicate with the exterior. I have not been able to 
follow exactly the way in which the complicated organ of the 
adult is developed from its comparatively simple beginning, as 
I have had to rely solely on sections. From these (figs. 23, 26, 
27, and 30) and others I have constructed a diagrammatic figure 
of the shape of the organ at the time of communication with 
the exterior (figs. 9 and 10). From the opening a narrow tube 
lined with quadrate epithelial cells goes forward and upwards 
(fig. 23) a short distance and then widens out into a spacious 
sac, which narrows again before reaching the fourth segment 
of the body. The cells of this portion are more columnar, and 
