§ 126. THE TURBELLARIA. 137 
CHAPTERS VI. AND VII. 
CIRCULATORY AND RESPIRATORY SYSTEMS. 
§ 126. 
As yet,only a very imperfect vascular system has been observed in the 
parenchyma of these animals. With the Dendrocoéli, there are constantly 
‘two principal vessels, extending along each side of the body, which give 
off many lateral branches and anastomose together at their two extremi- 
ties. 
This system has no central heart-like organ, and the walls of the vessels 
not being contractile, the circulation is probably effected through the gen- 
eral contractions of the body. The contained homogeneous and colorless 
liquid ought therefore to be considered as a nutritive fluid. 
With the Rhabdocoéli, the disposition is different. In many there are 
one or two vessels which traverse the body and loop at its extremities, 
without either giving off branches or diminishing in size. The movement 
of their colorless liquid is due to isolated vibratile lobules situated here and 
there in the vessels, 
This organization reminds one more of an aquiferous than a sanguineous 
system. 
Special respiratory organs are here wholly absent, if we do not regard as 
such the aquiferous system just mentioned. There remains, therefore, only the 
conjecture that the ciliary epithelium upon the entire surface of the body is 
subservient to a general cutaneous respiration, by constantly bringing the 
water in contact with the skin. 
1 Duges has described and figured very com- 
pletely the vascular system of Planaria (loc. cit. 
XV. p 160, Pl. V. fig. 1, 2, XXI. p. 85, Pl. IT. fig. 
24, 25). The cordiform organ which Mertens (loc. 
seit. p. 12, Taf. I. fig. 6, Taf. IL. fig. 3) refers to the 
vascular system of Planocera sargassicola and 
pellucida, is probably, as seen above, the central 
part of the nervous system. Dugeés is the only 
observer who affirms to have seen with the Den- 
rocoéli proper movements of the vessels; while 
Mertens, Ehrenberg (Abhandl. d. Berl. Akad. 
Joc. cit. p. 243), Schulze (loc. cit. p. 18), and Ors- 
ted (loc. cit. p. 16), have observed only the con- 
trary. 
2 With Derostomum leucops, Dug., I have 
seen two intertwined vessels of equal size through- 
out, extending from the caudal extremity to the 
head where they form a simple loop. At the cau- 
*([§ 126, note 2.) See for these two systems, 
Schmidt, Die Rhabdoc, Strudelw. &c., p. 11, and 
12* 
dal extremity, they approach so near to the cuta- 
neous surface that it is impossible to decide whether 
they terminate there by a loop, or open externally. 
Ehrenberg (Abhandl. d. Berl. Akad. loc. cit. p. 
178, Taf. I. fig. 2) has figured two pairs of such 
vessels with Gyratrix hermaphroditus, and which 
loop at the posterior extremity, but in front termi- 
nate indistinctly. The trembling in the interior of 
these vessels observed by Ehrenberg, indicates 
certainly the presence of vibratile lobules, and 
which Orsted (loc. cit. p. 17, Taf. III. fig. 48) has 
distinctly found in the vessels of Mesostomum 
Ehrenbergii, while Focke (loc. cit. p. 200) could 
see only their effects. These are the very vessels 
which this author supposes connect with the pha- 
rynx ; but this is not so according to my own ob- 
servations.* 
Neue Beitr. zur Naturgesch. d. Wiirmer, &c., 
p.15. -Ep. 
