258 MEDUSA OF THE WORLD. 



A medusa belonging to this genus was first described by Sars, 1835, under the name 

 Thaumantias multicirrata. In 1849, L. Agassiz founded for it the genus Tiaropsis, the best- 

 known species of which is Tiaropsis diademata of the New England coast, although the type 

 species is T . multicirrata of the northwestern coasts of Europe. 



GENERIC CHARACTERS. 



Eucopidae with 8 marginal sense-organs, 2 in each quadrant. Each sense-organ consists 

 of an entodermal ocellus and an open fold of velum, which contains a number of ectodermal 

 concretions. No cirri. The 4 gonads are developed on the 4 radial-canals. The manubrium 

 is set upon a short peduncle. 



The ocelli in Tiaropsis are situated within the entoderm of the circular canal. Each 

 ocellus consists of a cup-shaped mass of densely pigmented cells, the cavity of the cup being 

 filled with unpigmented protoplasm. The concavity of the eye is directed centripetally, as if 

 the eye were designed to see objects within the bell. Just below each ocellus is a large, pocket- 

 like, open fold of the velum within the cavity of which are a number of ectodermal concre- 

 tions, which are innervated by subumbrella or lower nerve-ring. 



The sense-organs of Tiaropsis are strikingly like those of Mitrocoma, but are more complex 

 in that they are provided with an entodermal eye, which is lacking in Mitrocoma. 



Figures showing sections of the sense-organs of Mitrocoma have been given by O. and R. 

 Hertwig, 1878 (Nervensyst. der Medusen, taf. 7, fig. 14) and by Maas (1895, Ergeb. der 

 Plankton Expedition, taf. 6, fig. 5). A cross-section of the sense-organ of Tiaropsis has been 

 given by A. Linko, 1900 (Trav. de la Soc. Imperiale des Nat., St. Petersbourg, tome 29, p. 154, 

 plate 29, fig. 5). 



The species of Tiaropsis are widely distributed, being found both in the Atlantic and 

 Pacific, and in tropical as well as temperate regions; but they are seen only near coasts and are 

 not creatures of the open ocean. There are two cohorts of the genus, one with many and 

 the other with few tentacles. 



Tiaropsis diademata L. Agassiz. 



Plate 31, fig. 11; plate 32, figs. 8 and 9. 



Tiaropsis diademata, Agassiz, L., 1849, Mem. Amer. Acad., vol. 4, p. 289, plate 6, figs. 1-16; plate 8, fig. 11.— Morch, 1857, 

 Beskriv af Gronland.— Agassiz, L., 1862, Cont. Nat. Hist. U. S., vol. 4, pp. 308, 355; figs. 45-48 in text, also plate 31, 

 figs. 9-15. — Agassiz, A., 1862, Proc. Boston Soc. Nat. Hist., vol. 9, p. 93, fig. 10; 1865, North Amer. Acal., p. 69, figs. 

 91-93. — Haeckel, 1879, Syst. der Medusen, p. 178. — Agassiz, A., and Mayer, 1899, Bull. Mus. Comp. Zool. at Harvard 

 College, vol. 32, p. 168, plate 7, fig. 23. — Linko ,1900, Trav.de la Soc. Imperiale des Nat., St. Petersbourg, torn. 29, p. 154, 

 planche 29, figs. T,a-b. — Nutting, 1901, Bull. U. S. Fish Commission, vol. 19, p. 381, fig. 100. — Hargitt, 1904, Bull. 

 U. S.Bureau of Fisheries, vol. 24, p. 49, 1 fig. 



Adult medusa (plate 31, fig. 11). — Bell about 15 mm. in diameter, flatter than a hemisphere. 

 Sides sloping, gelatinous substance of only moderate thickness. Tentacles very numerous 

 (150 to 200), short and slender, with well-developed basal bulbs. 8 adradial marginal sense- 

 organs, 2 in each quadrant; each consists of an entodermal mass of pigment, below which are 

 5 to 7 concretions contained in an open fold of the velum. The pigmented ocellus is situated 

 within the entoderm upon the inner sides of the circular vessel; the concretions are ectodermal 

 and inclosed within an open fold of velum (see Linko, 1900, p. 155, plate 29, fig. 5). Velum 

 well developed. There are 4 straight, narrow, radial tubes, and a simple, slender, circular 

 vessel. Stomach short and provided with 4 prominent crenated lips. There is a shallow 

 peduncle with widely flaring sides. The 4 gonads are linear and developed upon the 4 radial 

 tubes. They do not extend to the circular vessel, nor do they reach the manubrium. 



Entoderm of tentacle-bulbs and of stomach ocher-yellow. Gonads are cream-colored. 



Young medusa. — In the young medusa the bell is higher than a hemisphere and there is 

 a slight apical projection. The bell-walls are thin and tentacles much less numerous than in 

 the adult. The tentacles are covered with long, oval-shaped nematocyst-capsules that give 

 their surfaces a bristling appearance. There are 8 marginal sense-organs in medusae that have 

 only 24 tentacles and are only 2 mm. wide and 3 mm. high. Tentacles increase greatly in num- 

 ber, while sense-organs remain constant. Manubrium of young medusa is a simple, urn- 

 shaped tube having 4 prominent lips. During process of development bell becomes relatively 



