PLATE P. 



Fig. 1. Heart of Simocephalus vetulus. a, tendons attached to 

 lateral walls of heart, b, venous opening of heart, c, 

 muscular bands supporting the abdomen, connected by 

 transverse bands, d, cells of nutritive matter hiding the 

 arterial opening, e, thin membrane seen in section 

 which separates the venous from the arterial blood cur- 

 rents, is in focus near the side, but its situation in the 

 center is shown by the dotted line. Above this or out- 

 side it is the attachment of the powerful antennary 

 and mandibular muscles, f, posterior arterial sinus, g, 

 brood-sac. h, alimentary canal with thick glandular 

 cell walls, i, shell gland or excretory organ, j, power- 

 ful muscles supporting and moving the abdomen. 



Fig. 2. An early stage of the embryo of Daphnia schcefferi. a, 

 anus, n, nutritive globules or fat drops characteristic 

 of the summer embryo. m\ m\ outer and inner enve- 

 lope of the embryo. This is a nauplius stage, but not 

 the first or proper nauplius. The portion darkly shaded 

 is nutritive yolk. 



Fig. 3. A well advanced winter embryo of Z). scluefferi. a, shell 

 growing over the eyes. b. c, inner shell, d. outer shell, 

 e, lateral part of the head, f, antennules. g, labrum. 

 h, mandibles, i, maxilla, j, second maxilla ? k^, V, 

 m^, n^, branchial appendages of the 2d-5th pairs of feet, 

 represented by k, 1, m, n. o, first foot. p. antenna, 

 q, anus and intestine partly completed, s, shell growing 

 out from the maxillary region. 

 Older embryo bursting outer shell. 

 Egg after extrusion into the brood cavity. 

 Head of young embryo, a, lenses in formation, b, eyes 

 appearing as dark flecks, c, shell growing over the 

 head, d, labrum. e, antennule. 

 Longitudinal section through an ephippium. 

 Vertical section through an ephippial Daphnia schcefferi. 

 Somewhat oblique section through the ephippium (a, 

 b, c), heart (h), mandibles (m), and labrum (1). 



Fig. 10. A vertical section through the ephippium and its egg. 



