176 On the Heart and Circulation in the Pycnogonidse. 



Fiy. 10. B. Kladeiii, var. torosa, Jones: left valve. 11. Right valve. 

 12. Left valve. 



Fig. 13. B. lata, Vanuxem, sp. : right valve. 



Fig. 14. JB. Bussacensis, Jones : left valve. 



Fig. 15. B. Ribeiriana, Jones: left valve. 



Fig. 16. B. affinis, Jones: right valve. 



Fig. 17. B. Barrandiana, Jones: left valve. 



Fig. 18. B. strangulata, Salter : left valve. 



Fig. 19. B. strangulata, var. u : right valve. 



Fig. 20. B. strangulata, var. ^ : left valve. 21. Right valve (young). 



Fig. 22. B. strangulata, var. y : left valve. 



Fig. 23 a. B. hicornis, Jones : left valve. 23 b. Ventral asjject of the 

 same valve. 



Fig. 24. B. seminulura, Jones : left valve. 



Fig. 25. B. simplex, Jones : right valve. 



Fig.2Q. B. simplex 1 : right valve. 27- Right valve. 



Fig 28. B. mundula, Jones : right valve. 29 a. Left valve. 29 b. Ven- 

 tral aspect of part of the same valve. 30. Left valve. 31, Left 

 valve. 



XV. — On the Heart and Circulation in the Pycnogonidse. 

 By Dr. A. Krohn*. 



[With a Plate.] 



The opinion of Quatrefages that the Pycnogonidse are destitute 

 of a heart, has been contradicted by Zenker f^ who has succeeded 

 in recognizing this organ in Nymphon gracile. Zenker disco- 

 vered it in the position where analogy would lead us to expect 

 to meet with it ; he describes it as a very delicate sac, furnished 

 with ramified muscular fibres; its outlines may be most clearly 

 distinguished in the neighbourhood of the last pair of legs. I 

 had recently an opportunity, whilst examining a Phoxichilus 

 (probably F. spinosus), of convincing myself most decidedly of 

 the existence of the heart. I am thus enabled to furnish more 

 exact details as to its form and structure. 



The heart is a sac, of proportionably very large size, which is 

 placed, as in all the Arthropoda, in the back, above the intestinal 

 canal ; it is seen to extend from the hinder margin of the last 

 thoracic segment to the middle of the foremost one (PL VII. fig. 7). 

 W^hether it terminates at this point, or extends beyond it, must 

 remain undecided, as further observations are prevented by the 

 tubercle which rises on this part, upon the apex of which the 

 four eyes are situated. The heart is divided into three chambers 

 by two pairs of deep lateral notches or constrictions ; the hinder 



* From Wicgmann's Archiv for 1855, p. 6. 

 t Mailer's Archiv, 1852, p. 383. 



