422 THE ORIGIN OF VERTEBRATES 
were formed by the ventral part of the ccelom, and dipped originally 
into each appendage. We know also that each segment of an arachnid 
embryo possesses a ccelomic cavity in its ventral part which extends 
into the appendage on each side; this cavity afterwards disappears, 
and is said to leave no trace in the adult of any excretory coxal gland 
derived from its walls. If, however, it is found that in the very 
position where such organ ought to have been formed a segmentally 
arranged ductless gland is situated, the existence of which is shown 
by its taking up carmine, etc. then it seems to me that in all 
probability such gland is the modification of the original coxal gland. 
This is what Kowalewsky has done. Thus he states that 
Metschnikoff had fed Mysis with carmine-grains, and found tubules 
at the base of the thoracic feet coloured red with carmine. He him- 
self used an allied species, Parapodopsis cornutum, and found here 
also that the carmine was taken up by tubules situated in the basal 
segments of the feet. In Nebalia, feeding experiments with alizarin 
blue and carmine stained the antennal glands, and showed the 
existence of glands at the base of the eight thoracic feet. These 
glands resemble the foot-glands of Mysis, Parapodopsis, and Pale- 
mon, and lie in the space through which the blood passes from the 
thoracic feet, i.¢. from the gills, to the heart. In Squilla also, in 
addition to the shell-glands, special glands were discovered on the 
branchial feet on the path of the blood to the heart. These glands 
form continuous masses of cells which constitute large compact glands 
at the base of the branchial feet. Single cells of the same sort are 
found along the whole course of the branchial venous canal, right 
up to the pericardium. 
These observations show that the Crustacea possess not only true 
excretory organs in the shape of coxal glands, 2c. antennary glands, 
shell-glands, etc., in the cephalic region, but also a series of segmental 
glands situated at the base of the appendages, especially of the respi- 
ratory appendages: a system, that is to say, of coxal glands which 
have lost their excretory function, through having lost their external 
opening, but have not in consequence disappeared, but still remain 
im situ, and still retain an important excretory function, having 
become lymphatic glands containing leucocytes. Such glands are 
especially found in the branchial appendages, and are called branchial 
glands by Cuénot, who describes them for all Decapoda. 
Further, it is significant that the same method reveals the 
