MEM. M. C. Z., IX. No. 2. — ECHINODEEMATA. 29 



21. Gastnila, optical section ; u, digestive cavity. 48 hours. 



22. Gastnila, 68 hours old. 



23. Embryo 72 hours old ; the vasofieritoneal vesicle has sep.arated from the digestive cavity. 



24-27. EchinocarcUam cordatum. From E. Selenlca, Keiniblatter uud Organanlage der Echinideii, ISSO, PI. A' 1 1. 



Zcits, f. Wiss. Zool., XXXI II. 



24. Blastula, 30 hours ; a, position of the futui'e auus ; /, segmental oavity ; g, funnel-shaped <lepression in the 



ectodermal cells at the anal pole. 

 24'. Blastula showing the commencement of the invagination ; /«, 'nil, 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 ; /3, stomaidi ; 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 PI. X. figs. 14-33. 



Figs. 28-45 are devoted mainly to showing the formation of the arms of the Pluteus and the develojiment of the 

 young Arbacia. See also PI. X. figs. 32-44. 



28-37. Arbacia j>usliilosa. 



28. 29. Arbacia pustulosa. From W. Busch, Beobachtungen iiber Anatomic und Entwickelung einiger Wirbel- 



losen vSeethiere, 1851, PI. XIII. 



28. Young Pluteus, seen from the mouth side ; the oral extremity does not yet pi'oject 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 piiatn-lomi. From .Tohannes Miillei-, Ueber die Gattungen der Seeigellarven, Siebente Abhandlunf,', 

 (18.13,) I'ls. II., III., IV. Abhandl. der K. Akad. der Wiss. Berlin, 1855. 



a. Anus. o. rEsophagus. 



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 fi'om Busch (fig. 29). 



31. The same as fig. 30, seen in profile. 



32. Still older Pluteus, seen from the ventriil (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 pi-ojects 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 daik ]iigment-spots of the abactinal region of the young Ar- 



bacia are seen through the Pluteus membranes of tlie anal extremity ; the jiosterior 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, t, are 

 well developed, and pedicellaria; 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-shiipeil 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 pedicellariae are seen. 



38-45. Arbacia punctulata. 



38-40. Arbacia piwctulata. From J. W. Fewkes, On the Development of the Pluteus of Arbacia, 1881, PI. I. 

 Memoirs Peabody Academy of Science, Sixth Memoir. 



38-40. Young Plutei, showing the develojiment of the calcareous rods : ?», 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). 



