500 MEDUSA OF THE WORLD. 



been acquired independently of and is not homologous with the bell of the Leptolina medusae. 

 (See Goette, 1907, Zeit. fur wissen. Zool., Bd. 87, p. 289.) 



The peculiar velum of the Carybdeidae among the Scyphomedusae is a structure of the 

 subumbrella, not of both subumbrella and exumbrella as in Hydromedusae. It may be regarded 

 as a parallelism, and not genetically related to the velum of Hydromedusae. The exumbrella 

 nerve-ring, found commonly in Hydromedusae, does not exist in the Scyphomedusae, but is 

 replaced by a subumbrella plexus of fibers extending between the marginal sense-organs and 

 also radially inward from these ganghomc centers. There is also a diffuse, nervous, epithelial, 

 ectodermal network over the subumbrella. The subumbrella alone is sensitive to stimuli, the 

 exumbrella being non-sensory, but covered with a nematocyst-bearing epithelium. 



In the Scyphomedusae the mature, sexual products are found in the entoderm, whereas 

 they are usually in the ectoderm in Hydromedusae. The velum which is universally present 

 in Hydromedusae is absent in Scyphomedusae. The 4 interradial, gastric septa which are 

 always seen in the scyphostoma larva of Scyphomedusae are not found in the polyp stage of 

 Hydromedusae. 



When we come to consider the relationships of the several orders constituting the Scypho- 

 medusae themselves we meet with difficulties which render our classification only tentative. 

 The history of the attempts to classify the Scyphomedusae have recently been reviewed by Maas, 

 1907 (Ergeb. und Fortschritte derZool., Bd. I, p. 189), and by Bigelow, 1909 (Mem. Museum 

 Comp. Zool. at Harvard College, vol. 37, p. 13). Marked advances over Haeckel's artificial 

 classification were made by Claus, 1878, 1883, who showed that the Carybdeidae are a very 

 aberrant group not closely related to other Scyphomedusae, and not descended from the Stauro- 

 medusae as Haeckel believed. As Claus showed, the interradial, gastric septa in Carybdeidae 

 may be newly arisen fusions of the entodermal walls of the exumbrella and subumbrella and 

 not derived from the interradial taeniolae of the scyphostoma. Unfortunately, however, no one 

 has seen the late scyphostoma stage in Carybdeidae, and hence our knowledge of their rela- 

 tionships must remain doubtful. Claus, 1883, and VanhofFen, 1892, called attention to the 

 supposed importance of the presence or absence of the interradial, gastric septa in the classi- 

 fication of sexually mature Scyphomedusae but we must remember that these are absent in 

 Semaeostomae and found in Coronatae, yet these two orders are closely related. Moreover, 

 interradial gastric septa are found both in Stauromedusae and Carybdeidae yet all modern 

 students agree that these groups are widely divergent. The sessile Stauromedusae certainly 

 approach very closely in structure to the type of the scyphostoma, but this may be the result 

 of degeneracy due to their sedentary habit. 



VanhofFen, 1892, in a masterly paper called attention to the importance of the annular 

 furrow and marginal, exumbrella sculpturing of the bell in certain of Haeckel's Discome- 

 dusae, and he grouped these under the designation Coronatae. Claus, 1883, had previously 

 shown that in Naiisitlwe, which is a typical member of the Coronatae, the interradial septa of 

 the central stomach are derived from the 4 taemolae of the scyphostoma. These septa are, how- 

 ever, absent in the medusa-stage in Semaeostomeae and Rhizostomae, although they are found 

 in the scyphostoma. 



All evidence both anatomical and embryological shows that the Rhizostomae are descended 

 directly from the more simply organized Semaeostomeae. It is probable that the Coronatae and 

 Semaeostomeae are not descended one from the other, but are derived from some common 

 ancestral stock which has disappeared. The Stauromedusae may represent a highly specialized 

 derivative from this common ancestral form, their specialization being due to their sessile, 

 attached habit of life. The Carybdeidae are so aberrant and our knowledge of their develop- 

 ment is so imperfect that we have not yet been able to determine their relationships to other 

 Scyphomedusae. 



The habits and distribution of the several orders of Scyphomedusae are characteristic. 

 The Stauromedusae are found only in the Arctic and Antarctic regions and in cold seas, 

 and are unknown from the tropics; thus furnishing us with an interesting problem in dis- 

 tribution. The planula is not pelagic but creeping and is devoid of cilia, and the medusae 

 are sessile or creeping, and do not pulsate rhythmically. They affix themselves to seaweeds 

 and rocks and rarely move from their places of attachment. They are probably degenerate 

 forms although their relationship to other Scyphomedusae is uncertain. They are confined to 

 the shores, but may be carried far from their normal habitats upon drifting seaweed. 



