42 2 THE ORIGIN OF VERTEBRATES 



were formed by the ventral part of the coelom, and dipped originally 

 into each appendage. We know also that each segment of an arachnid 

 embryo possesses a crelomic 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 My sis 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 Pake- 

 mon, and lie in the space through which the blood passes from the 

 thoracic feet, i.e. 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, i.e. 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 

 in 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 Cuenot, who describes them for all Decapoda. 



Further, it is significant that the same method reveals the 



