THE DEVELOPMENT OF ADMETUS PUMIUO. 623 



The end of the midgut is not in open communication with 

 the rectum, being closed by a plug of entoderm cells. 



The rectum is formed by an invagiuatiou of the ectoderm. 

 At its exterior end its cells quite resemble those forming the 

 epidermis ; at its interior (proximal) end the cells are vacuo- 

 larised. The rectum, as also the most anterior parts of the 

 dilators, canal, is supplied with powerful muscles ; one set, the 

 alimentary, runs from its walls to the skin of the abdomen ; the 

 contractors being ring-shaped, as in the pharynx and muscle- 

 stomach. 



As in Scorpions the Malpighian tubes are without doubt of 

 entodermatic origin, as they enter the alimentary canal near 

 the posterior end of the midgut. In this stage they are 

 already well developed ; they are very long and run parallel 

 and juxta-apposed to the alimentai'y canal. 



8. The Heart. — The heart of the Pedipalpi has been best 

 described by Pereyaslawzewa as yet. 



The heart at this stage consists of a long tube, lying 

 dorsally, immediately beneath the skin, in the median plane 

 of the abdomen. Its walls are thick, but do not contain 

 many muscle elements. 



At each segment the heart widens, and seems to me to 

 give off a pair of small arteries. A large artery leaves the 

 heart at its anterior end, this runs into the cephalothorax; 

 following the outer surface of the embryo and reaching the 

 cerebral ganglion it suddenly bends downwards; soon after- 

 wards it divides into two branches, which run forwards on 

 both sides of the muscle-stomach till they reach the central 

 nervous system, when they terminate abruptly. 



My embryos not being so advanced as Pereyaslawzewa's 

 (37), I am not in a position to state anything about the other 

 arteries and veins which she has seen ; at the same time I 

 consider her statement that the heart terminates ''par une 

 artere post-abdominale " as at all events not perfectly 

 correct, since, as is hardly necessary to state, the Phryniscid^ 

 have no post-abdomen, either as embryos or as adults. 



Inside the heart the blood-cells are to be seen. These are 



