PECULIAR HABITAT OF A PYCNOGONID, 20I 



of the rate of the heart beat of Endeis spinosus which I made at 

 Woods Hole, September 4, 1904, showed 172 contractions per 

 minute, the rate slowing down in a short time after the animal 

 had been mounted under a cover glass. The next morning other 

 specimens showed a rate of 126 to 136 beats per minute. Fig. i, 

 from a rough sketch made at the time, shows the position and 

 shape of the heart ; the dotted lines indicate the apparent change 

 in shape as the muscular side walls contract, and the arrows show 

 the direction of observable streaming of the corpuscles in the 

 body fluid or blood. It must be remembered that owing to the 

 attachment of the side walls to the dorsal integument they must 

 be stationary at this point, the contraction of the muscles in the 

 side walls tending to draw the sides together further down, thus 

 reducing the capacity of the enclosed space (see Fig. 2). It is 

 probable that with the alternate contraction and relaxation of 

 the heart the transverse septum {sept.), which divides the body 

 space into dorsal and ventral chambers, is raised (Fig. 2, sept.) 

 and lowered to some extent, which would help to force the blood 

 out into the legs at each systole and to draw blood from the legs 

 into the pericardial space at the diastole. 



The two pairs of lateral ostia, opposite the lateral processes 

 of the body for the attachment of the second and third pairs of 

 legs respectively, could be plainly distinguished ; the single open- 

 ing at the posterior end was somewhat less distinct. The stream- 

 ing of the blood cephalad through this portion of the heart left 

 no doubt, however, of the existence of such an opening, except 

 in one case in which the blood could be seen to move back and 

 forth in this region without a definite streaming forward. This 

 would seem to confirm Dohrn's observation that in some cases 

 there is no posterior terminal ostium, or at least it appears that 

 if it existed in this instance it was closed and not functioning. 



Although, on the whole, there appeared to be a real circulation 

 from the body out into the legs and back, this was rendered more 

 or less indefinite by the peristaltic contractions of the intestine, 

 which imparted a sort of churning motion to the blood and kept 

 it moving back and forth. This was especially evident in the 

 more expanded femoral joints; in the basal parts of the leg and 

 in the tibia a more definite streaming could be observed. The 



