PECULIAR HABITAT OF A PYCNOGONID. 1 99 



the matter with the statement that there exists "une circulation 

 vague." According to de Quatrefages (1845, pp. 75, jG) no 

 heart or vascular system exists, but the blood is agitated back 

 and forth in part by the movements of the legs and also by the 

 muscular movements of the intestine, which he believed to lie 

 freely in the internal space of the legs. Van Beneden (1846, pp. 

 72, 73), observing a living Nymphon, was the first to make out 

 a regular circulation. He states that the blood may be seen to 

 flow down one side of a leg and back the other, then into the 

 next following leg, and so on to the last pair, after which he 

 could not tell what course it takes. On account of the opacity of 

 the intestine he could not determine whether there was a heart 

 or dorsal vessel present, but he observed a contractile membrane 

 at the bases of the legs. (It is probable that what he saw were 

 the valves of the heart where that organ extended beyond the 

 outline of the intestine.) 



Zenker (1852, pp. 382, 383), also studying Nymphon, was 

 apparently the first really to see the heart. He describes it as a 

 thin-walled sac with ramifying muscle fibers, the contour being 

 most clearly discernible in the region of the last pair of legs. 

 Three years later Krohn (1855) published a much fuller descrip- 

 tion of the circulation, together with a figure of the heart of 

 Endeis spinosus. Hoek (i88ia, 1881&) described the structure 

 of the heart in Colossendeis and certain other forms. According 

 to him there are ordinarily three pairs of lateral ostia (the pos- 

 terior pair being very close together) except in Pallene hrevirostris, 

 which has but two pairs. In the same year Dohrn (1881) pub- 

 lished the best description of the structure of the heart and of the 

 circulation that has yet been given, his description being based 

 principally again on Endeis. He found here two pairs of lateral 

 ostia, with commonly, but not always, an unpaired aperture at 

 the posterior end. There are no blood vessels aside from the 

 heart, but the blood driven out from the anterior end of the 

 heart is forced into the proboscis and runs back along the ventral 

 side of the body, and from here it flows out into the legs. In 

 general the direction of flow is centrifugal on the ventral side 

 and centripetal on the dorsal side (except in the proboscis, where 

 it is just the reverse). A thin membrane, which supports the 



