30 TWELFTH ANNUAL REPORT. 



covered by the muscles of the mandibles and obscured by the 

 many supporting and vibrating membranes alluded to. 



It is, however, suspended by two folds of membrane which I have 

 been inclined at times to believe blood-vessels through appearances 

 resulting from the confused currents flowing about them. The 

 upper margin is also attached by a pair of cords directly to the 

 superior part of the shell. The anterior opening or arterial valve 

 is most perplexing, and the following description which applies 

 only to Daphnia schsefFeri mjist be subject to some doubt. It ap- 

 pears however that it has been in a measure misunderstood by pre- 

 vious writers, and namely by Glaus, who compares it with that of 

 Leptodora, which if correctly described by Weismann, is not at all 

 identical in form, but quite comparable with one of the sides or lips 

 of the venous opening. It does not seem to be connected by a 

 thread, as stated for Leptodora, with the aortal bulb, for in reality 

 there is no aortal bulb; the heart simply is connected with the 

 system of membranes which more or less inclose the system. The 

 floor of the so-called aortal space is a membrane which separates the 

 outflowing stream from a current which flows toward the abdomen 

 and passes directly under the arterial opening, so that it appears as 

 though there was a stream entering the heart from before as well 

 jji j as at the sides; the arterial opening being nearer the dorsal part of 



ii' I the heart than is naturally expected, and the slight enlargements 



at the attachment of the supporting membrane favoring the im- 

 pression that there is here a veritable opening. The out-flowing 

 blood stream is bounded at first by the membrane above mentioned, 

 which is farther on reflexed onto the shell and ^intestine so that 

 the streams in the head flowing just under the shell are separated 

 from the deep dorsal stream flowing from the heart.* This main 

 current passes to the region of the eye between the horns of the 

 caeca of the alimentary canal, and thence beneath the stomach, and 

 here divides, part becoming external and a deeper part passing un- 

 der the intestine, thence in front of the heart, flows into the deep 

 sinus which, as before said, passes beneath this organ. Other por- 

 tions of the returning stream flow around the angle of the union 

 of the head and body and constitute a stream just above the feet 

 in which the current flows vigorously. 



Yet other portions flow into the region of the shell-gland and 

 are united with blood which here parses through the numerous 

 sinuses described by Glaus as surrounding this organ (Die Schalen- 

 11 I 



ij j *In Pasithea rectirostris this septum is easily seen as a swaying membrane, which near 



iij 1 the 'eye is reflexed to the top of the shell. 



i.i 



