458 MEDUSAE OF THE WORLD. 



The color of S. mediterranea is very variable. Some specimens are nearly colorless, the 

 stomach and tentacles being slightly milky or yellow. Others show these parts intense yellow- 

 green by reflected, or red by transmitted, light. 



S. mediterranea is widely distributed, although rarely very common, except in the Mediter- 

 ranean, where it is abundant. It is found in the tropical Atlantic, Pacific, and Indian Oceans, 

 and in the Mediterranean. I have figured it in the Fiji Islands, South Pacific, and at Tortugas, 

 Florida, and can detect no specific differences between specimens from these widely-separated 

 places. The medusa appears in small swarms at or near the surface. Maas, 1906, records this 

 medusa from the Antarctic Ocean, about S. lat. 70°, W. long. 8o° to 90 . 



The development has been studied by J. Miiller, Metschnikoff, Maas, and Woltereck. 

 The medusa develops directly from an actinula larva, each egg developing into a single medusa. 

 According to Metschnikoff, 1886, the egg is 0.23 mm. in diameter and is cast out into the water 

 between 5 and 6 in the afternoon from February to December, in the Mediterranean. Segmen- 

 tation is total and equal until the 16-cell stage is reached; then the entoderm is formed by multi- 

 polar ingression. Cells migrate into the interior from any point whatsoever, and these soon 

 fill up the central segmentation cavity and form the entoderm. The larva then elongates and 

 the ectoderm becomes ciliated. Soon the elongation becomes so great that the larva resembles 

 a detached tentacle of some hydroid, it is so very long and spindle-shaped. The entodermal 

 cells of both ends are disk-like and set in a single row, while in the center the entoderm forms a 

 parenchyma-like mass. The 2 elongated ends become the tentacles and the center develops 

 into the body of the actinula larva. The actinula is at first miter-shaped, with two long, stiff, 

 filiform tentacles 108 apart. The entoderm of the body then consists of a mass of vacuolated 

 cells and is entirely incased by the ectoderm. Later the mouth breaks through and the 

 central stomach-cavity develops, lined by a single layer of entodermal cells. At first there 

 is no gelatinous substance between ectoderm and entoderm, but this begins to form above the 

 stomach and is probably secreted by the entoderm. It forms a thick, lenticular mass between 

 the ectoderm and entoderm. No peripheral canals or ring-canal are developed. The bell- 

 walls growout from the sides of the larva after the tentacles and central stomach have developed. 



Woltereck finds that when the actinula larva has a pair of tentacles and has developed its 

 mouth, although as yet there is no gelatinous substance, there is a cluster of elongated, cylin- 

 drical, epithelial cells at the aboral pole of the larva and these bear long cilia. He found 

 these cells persisting in a young medusa with well-developed bell and with its gelatinous 

 substance well formed, which was dredged from deep water off Villafranche, Mediterranean. 

 Woltereck finds also that there is a well-developed, ciliated, aboral pole-plate encircled by a 

 ring-like ridge in the actinula larva of Tubularia. No trace of an aboral sense-organ has been 

 seen in adult specimens of Solmundella bitentaculata or S. mediterranea, but it will be recalled 

 that a pair of apical lithocyst-clubs are described by Dawydoff in his remarkable form Hydroc- 

 tena. The actinula larva of Solmundella is transformed into the medusa by the growing out 

 of the bell from the sides of the larva, and it is interesting to observe that the tentacles of the 

 medusa are those of the Actinula and are formed long before the bell begins to develop. Sol- 

 mundella may be regarded as a medusiform actinula and its bell is not homologous with the 

 bell of the Anthomedusae and Leptomedusae. The medusa-shape has probably been acquired 

 independently in Trachylina and in Leptolina medusae. 



Genus HYDROCTENA Dawydoff, 1903. 



Hydrociena, Dawydoff, 1903, Zool. Anzeiger, Jahrg. 27, p. 223; 1903, Mem. Acad. Imperiale des Sci. St. Petersbourg, ser. 



8, vol. 14, No. 9. 

 Hydracienn, Woltereck, 1905, Vcrhandl. Deutsch. Zool. Gesell., 15 Jahresvers., p. 115; Zool. Anzeiger, Jahrg. 28, 1904. 



The type species and only representative of the genus is Hydrociena salensku, described 

 by Dawydoff from the Malay Archipelago. Dawydoff" is the only student who has observed 

 this remarkable medusa. 



GENERIC CHARACTERS. 



Narcomedusae with an apical sense-organ consisting of sensory-clubs and ciliated ecto- 

 dermal epithelium and with 2 diametrically opposite tentacles arising from sheath-like sockets 

 in the sides of the bell. The axial cores of these tentacles are said to consist of gelatinous 



