No.2.] THE MESODERM AND MESENCHYM. 359 
ben, ihren Ursprung nehmen”’; they stand in open communi- 
cation with the pseudocoelic body sinuses, the latter having an 
endothelial lining in the Rhynchobdellini, but not in the Gnath- 
obdellini. The blood corpuscles of the leeches are probably 
derived from the blood-vessels as well as from the so-called 
“‘parenchym” (Lankester). In the Sipunculacea, there is 
probably a communication between the blood-vessels and the 
coelom, the splanchnopleuric peritoneum of the latter being 
ciliated, as in the frog (Lang, p. 213). 
Echinodermata: ‘‘Spaltbildungen im Mesenchym sollen die 
Blutgefasse der Holothurie entstehen lassen. . . . Die Blut- 
zellen hingegen sollen sich von den Wandungen des Hydro- 
enterocoels losgelost und bei der Bildung jener Gefasse 
betheiligt haben. Diese freien Zellen, welche sich sowohl 
in der Leibeshohle, wie in den Ambulacral- und Blutgefassen 
finden, wiirden also nach dieser Auffassung (Semon) nicht von 
dem urspriinglichen Mesenchym abstammen” (Korschelt and 
Heider, p. 286). In the Asteroidea: “In der zwischen Hydro- 
coel-, Enterocoel-- und Darmwand gelegenen Mesenchym- 
schicht bildet sich dort ein Spalt, welcher eine Auskleidung von 
sehr flachen Zellen aufweist”’ (zbzd., p. 290). The blood-vessels 
in the Crinoidea communicate with the coelom, and arise as 
cavities in proliferations of the enterocoel (zdzd., p. 302). 
In all the Arthropoda, as is well known, the blood-vessels 
stand in communication with the body cavities. The heart of 
the Crustacea is mesodermal in origin, its cavity archicoelic 
(Korschelt and Heider, p. 376). In Limulus (zdzd., p. 528), 
the heart develops from the mesodermal plates, and cells wan- 
der into its cavity, which had their origin in its walls. In the 
Scorpionidea (zbzd., p. 556), a segmented coelomic cavity arises 
in the primitive mesodermal proliferation, the walls and free 
corpuscles of the blood-vessels being mesodermal (is their cav- 
ity then coelomic ?). As to the Araneina (zézd., p. 615), it isa 
disputed point whether the blood corpuscles are merocytes or 
whether they originate from the protovertebrae ; later they form 
a solid chord on the dorsal side of the embryo; they separate 
from one another again, when the heart becomes formed by a 
coalescence of neighboring splanchnopleuric layers; thus the 
