§ 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 Dendrocoeli, there are constantly 

 two principal vessels, extending along each side of the body, which give 

 oflf 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 Rhabdocoeli, 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 entii-e surface of the body is 

 subservient to a general cutaneous respiration, by constantly bringing the 

 water in contact with the skin. 



1 Dugis has described and figured very com- dal extremity, they approach so near to the cuta- 

 pletely the vascular system of Planaria (loc. cit. neous surface that it is impossible to decide whether 

 XV. p 160, PI. V. fig. 1, 2, XXI. p. 8.5, PI. n. fig. they terminate there by a loop, or open externally. 

 24,25). The cordiform organ which 3/er<ens (loc. £Aren6erg- (Abhandl. d. Bei-1. Akad. loc. cit. p. 

 cit. p. 12, Taf. I. fig. 6, Taf. II. fig. 3) refers to the 178, Taf. I. fig. 2) has figured two pairs of such 

 vascular system of Planocera sargassicola and vessels with Gyratrix hermapkroditus, and which 

 pellucida, is probably, as seen above, the central loop at the posterior extremity, but in front termi- 

 part of the nervous system. Dug-es is the only nate indistinctly. The trembling in the interior of 

 observer who afiirms to have seen with the Den- these vessels observed by Ehrenberg, indicates 

 drocoSli proper movements of the vessels ; while certainly the presence of vibratile lobules, and 

 Mertens, Ehrenberg (Abhandl. d. Berl. Akad. which Orsted (loc. cit. p. IT, Taf. III. fig. 48) has 

 loc. cit. p. 243), Schulze (loc. cit. p. 18), and Ors- distinctly found in the vessels of Mesostomum 

 ted (loc. cit. p. 16), have observed only the con- Ekrenbergii, while Focke (loc. cit. p. 200) could 

 trary. see only their effects. These are the very vessels 



2 With Derostomum, leucops, Dug., I have which this author supposes connect with the pha- 

 seen two intertwined vessels of equal size through- rynx ; but this is not so according to my own ob- 

 out, extending from the caudal extremity to the servations.* 



head where they form a simple loop. At the cau- 



* [ § 126, note 2.] See for these two systems, Neue Beitr. zur Naturgesch. d. Wurmer, &c., 

 Schmidt, Die Rhabdoc, Strudelw. &c., p. 11, and p. 15. — Ed. 



12^ 



