MEM. M. C. Z., IX. No. 2.— ECHINODERMATA, 29 
21. Gastrula, optical section ; u, digestive cavity. 48 hours. 
22. Gastrula, 68 hours old. 
23. Embryo 72 hours old ; the vasoperitoneal vesicle has separated from the digestive cavity. 
24-27. Echinocardium cordatum. From E. Selenka, Keimblatter und Organanlage der Echiniden, 1880, Pl. VII. 
Zeits. f. Wiss. Zool., XX XIII. 
24. Blastula, 30 hours ; @, position of the future anus ; 7, segmental cavity ; g, funnel-shaped depression in the 
ectodermal cells at the anal pole. 
24!, Blastula showing the commencement of the invagination ; m, m’, clusters of cells of the mesoderm which have 
become separated from the ectoderm at the anal pole. 
25. Gastrula 40 hours hold. 
26. Longitudinal optical section of the same. 
27. Young larva 50 hours old, optical section ; the vasoperitoneal sac has become separated from the digestive 
cavity ; this is now differentiated into the a, cesophagus ; 8, stomach ; and y, the intestine. 
Figs. 1-27 are grouped together to show, in the earlier stages of the Pluteus of several Echini types, the forma- 
tion of the mesoderm cells, the invaginations of the gastrula, the development of the vasoperitoneal sacs, and the 
differentiation of the original digestive cavity of the gastrula. See also Pl. X. figs. 14-33. 
Figs. 28-45 are devoted mainly to showing the formation of the arms of the Pluteus and the development of the 
young Arbacia. See also Pl. X. figs. 32-44. 
28-37. Arbacia pustulosa. 
28, 29. Arbacia pustulosa. From W. Busch, Beobachtungen iiber Anatomie und Entwickelung einiger Wirbel- 
losen Seethiere, 1851, Pl. XIII. 
28. Young Pluteus, seen from the mouth side ; the oral extremity does not yet project beyond the level of the fold 
connecting the dorsal anal arms. 
29. Somewhat more advanced than the preceding stage, seen from the dorsal side; the oral extremity projects 
nearly as much as the dorsal arms. 
30-37. Arbacia pustulosa. From Johannes Miiller, Ueber die Gattungen der Seeigellarven, Siebente Abhandlung, 
(1853,) Pls. IL, UL., 1V. Abhandl. der K. Akad. der Wiss. Berlin, 1855. 
a. Anus. o, (Esophagus. 
d. Digestive cavity (stomach). t. Tentacular lobes of water-system. 
om. Mouth. 
30. Young Pluteus seen from the ventral side ; somewhat older than the oldest stage copied from Busch (fig. 29). 
31. The same as fig. 30, seen in profile. 
32. Still older Pluteus, seen from the ventral (mouth) side ; the oral arms, mere knobs in the preceding stages (figs. 
30, 31), have greatly increased in length ; first trace of the posterior anal pair of arms. 
33. Still older Pluteus, seen from the dorsal side. The posterior pair of anal arms projects well beyond the general 
outline of the anal extremity ; the auricles are beginning to form as folds of the vibratile cord, between the 
dorsal and oral arms, and the second pair of oral arms is present as mere knobs at the base of the oral 
arms. 
34. Still older Pluteus, seen from the mouth side ; the dark pigment-spots of the abactinal region of the young Ar- 
bacia are seen through the Pluteus membranes of the anal extremity ; the posterior pair of anal dorsal arms 
has grown rapidly since the last stage (fig. 33), exceeding in length the oral arms; the second pair of 
dorsal arms is also present, equalling in length the oral arms ; the auricles are also well developed. 
35. Fully developed Pluteus, with two pairs of oral arms and a smaller pair of dorsal oral arms, with large auricular 
arms, and the long median dorsal arms of equal length ; the tentacular lobes of the water system, ¢, are 
well developed, and pedicellariz even have made their appearance on the surface of the young Arbacia. 
In this stage the young Arbacia is about to resorb the Pluteus. 
36, 37. Young stages of Arbacia ; 36 seen from above, 37 somewhat in profile. The club-shaped spines are the 
young spines of the edge of the test of the young Arbacia; the straight spines are the remnants of the 
Pluteus rods in process of atrophy and resorption. In the stage 37, young pedicellariz are seen. 
38-45. Arbacia punctulata. 
38-40. Arhacia punctulata. From J. W. Fewkes, On the Development of the Pluteus of Arbacia, 1881, Pl. I. 
Memoirs Peabody Academy of Science, Sixth Memoir. 
38-40. Young Plutei, showing the development of the calcareous rods: m, mouth; d, digestive cavity. Some- 
what younger than the stages of Arbacia figured by Busch (figs. 28, 29), and somewhat older than those 
given by Selenka (figs. 22, 23). 
