MEM. M. C. Z., IX. No. 3.—ACALEPHS, POLYPS. 23 
wall or membrane (m.) stretched over two vertical plates, which are parallel, and united by horizontal 
floors (8.). The lowest of the chambers between two floors (a) is a float. y. Chambers (seven) situated 
between horizontal curved partitions in the body of the disk. hy. Liver? (brown cells). 6. Disk margin. 
pyt. Polypite. 
6, 7. Rataria, from one side. pyt. Polypite. 6. Disk rim. 2. Prehensile tentacles (hydrocysts 2). ¢. Sail. 
Notice the characteristic vertical lines on external wall of the sail. The form of the sail, although unlike 
that of fig. 2, is not very distant from that of a Vele/7u a little younger. 
8, 9. From Agassiz, op. cit., Pl. II. fig. 10; Pl. VIII. fig. 8. 
8. Medusa of Porpita. wbr. Umbrella. 
9. Medusa of Velella. 7. th. Radial tubes with yellow cells. wbr. Umbrella. ta. Tentacle. Rows of nemato- 
eysts are seen on the surface of the bell. 
10-19. From Metschnikoff, Studien tiber die Entwickelung der Medusen und Siphonophoren. Zeit. f. Wiss. Zvil. 
Bd. XXIV., Pl. VI. figs. 5; 7, 8, 9; Pl. VII. figs. 11, 14, 16; Vl. XI. figs. 6, 8; Pl. XII. fig. 9. 
10-15. ELpibulia aurantiaca. 
10. Planula with interior filled with spongy cells, and an epiblastic cap (¢b.) on the pointed pole.- a. is arbi- 
trarily taken as the anterior pole. ¢/. Spongy mass of cells. 
11. Older embryo, in which a nectocalyx and tentacle have begun to form, The epiblast (¢ b.) rises in two promi- 
nences, the smaller of which is the bud of a tentacle (/7.), and the other a nectocalyx (nex.) Under the 
epiblast is another layer, probably the hypoblast. The epiblast forms the lining of a cavity (the bell 
eavity). Cilia (c7/.) still remain over the whole body. The whole vitellus (v/.) is taken up by the spongy 
maass of cells. 
12. An older embryo than the last. Although this is a faithful copy of Metschnikoff’s figure, it is, like his, 
believed to be faulty in this particular. The course of the epiblast, after it leaves the lower surface of the 
bell, on the side turned to the observer, probably passes by reflection into the walls of the tentacle, instead 
of to the upper pole of the embryo. The hypoblast closely covers the infolded part of the epiblast in the 
bell, and extends, also, into the tentacles. It also forms a loop (so cy.), the future somatocyst. A 
gelatinous layer also appears between hypoblast and external epiblast in the nectocalyx. Whole swrface 
ciliated. 
13. Older larva, with the primary nectocalyx of considerable size. eb. Epiblast which lines the bell cavity (7.) 
and is also stretched ‘over the whole ovum. fb. A layer of hypoblast in the bell, At socy. this hypoblast 
forms a loop, the future somatocyst. Between it (db.) and the epiblast of the surface of the bell is a 
gelatinous layer (?it mesoblast.) 26. Swollen hypoblastic cells. vt. Vitellus with spongy cell mass. 
The large projection lined by cells (2 6!.) becomes later a polypite. Epibulia, like Crystallodes, absorbs 
the vitellus, which is not directly changed into the polypite, as in some species of Agalma. 
14. Older larva (primitive larva). pyf. Polypite with two layers best marked near the pointed end. socy. Soma- 
toeyst. ubr. Umbrella. vf. Vitellus. 
15. Oldest larva, in which the vitellus is wholly absorbed, and a small hydrophyllium (hy ph.) has formed in its 
place. A second nectocalyx (a. ne #.) has formed, which, from its future position, is called the anterior. 
It is regarded as homologous with a. nex. of figures 30, 31. The distal end of the polypite (pyt.) is open, 
forming a mouth. sacy. Somatoeyst. Although the intermediate larve between this and the adult Epi- 
bulia ave not known, it is probable that the larger nectocalyx, with the somatocyst, is transitory, and is 
later lost. It can then be supposed that fig. 15 corresponds with a Monophyes larva, or a detinite stage 
(fig. 29) in the cyclical development of Muggica. 
16-17. Gleba hippopus. Forsk. 
16. Embryo, with first nectocalyx beginning to form. Around the whole embryo is stretched an epiblast (e b.). 
The infolded portion of the epiblast (¢4!.) forms the inner bell wall. The hypoblast (40.) is found just 
below these two. Whole remainder cf the vitellus taken up by a spongy mass of cells, which closely 
resemble the ‘‘ fatty cells” of Clenophora and other ccelenterate larvie. The bell cavity probably forms by 
a dissolution of epiblastie cells (¢ 0'.). 
17. Larva, with a helmet-shaped bell (ubr.), fully formed. This stage may be called the primitive larva or 
Monophyes stage, and is homologous-with figs. 14 and 29. The helmet-shaped bell is probably provisional. 
eb. Epiblast. 46. Hypoblast. r. tb. Radial tube. vt. Vitellus. 
18. Halistenma rubrum. From Metschnikoff, op. cit., Pl. X. fig. 6. 
Larva with primitive nectocalyx (xe z.) beginning to form, showing also the young pneumatocyst (pncy.). In 
larve younger than this we have two layers — epiblast and hypoblast — formed first ; then an elevation of 
both, by which a cavity is left between the hypoblast and the vitellus. The first structure formed is a bell 
(ncx.) ; the second (pncy.), a pneumatoeyst ; and the third (gm.), probably a tentacle. ‘The pneumatocyst 
