HYDROMEDUSAE. 723 



INTRODUCTION. 



Although the collection does not contain many specimens, a few more would have been 

 a distinct advantage, and the species are not numerous; it has, nevertheless, brought to light 

 some interesting medusae. It is a welcome addition to our knowledge of the Hydromedusae 

 of the Indian Ocean, and I express my sincere thanks to Mr Stanley Gardiner for allowing 

 me the privilege of examining the specimens. 



The Anthomedusae and the Leptomedusae of the Indian Ocean are not well known and 

 a very few species have been recorded. There are five species in this collection and four 

 of them are new to science. The medusae belonging to these two Orders are usually found 

 in littoral waters and seldom far away from land. They are liberated, with few exceptions, 

 from fixed hydroids, and consequently their geographical distribution is limited to the region 

 occupied by their hydroids. A genus often has a wide geographical range, but seldom its 

 species, which are usually confined to definite areas, or even to certain localities, so that some 

 of the new species belonging to the Anthomedusae and the Leptomedusae are probably 

 limited to the Maldive Islands. 



The Trachomedusae, the Narcomedusae and the Siphonophora have no fixed stage in their 

 life-history and they are the inhabitants of the oceans, drifting hither and thither with the 

 currents. Their species have usually a very wide geograj^hical range and some extend over 

 the Atlantic, Indian, and Pacific Oceans. Although Mr Gardiner made extensive use of the 

 tow-net, yet a very few specimens of the Oceanic medusae were taken, and considering the 

 geogi'aphical position of the Islands one would have expected to have seen a more extensive 

 collection. 



SUMMARY. 



In the revision of the Williadae I have used for the generic character the number of 

 radial canals which leave the stomach, and for the specific character the branching of the 

 canal system. There are now two genera, namely, Prohoscidactyla and Willia. The genera 

 Byscannota and Willetta of Haeckel are no longer needed. In three species of the Williadae 

 I have found that the circular canal is absent and that its place is occupied by a solid 

 chord of endoderm cells. The radial canals are in direct communication with the basal bulbs 

 of the tentacles. 



Mesonema pensile, one of the Aequoridae, has the lower wall of the stomach quite rudi- 

 mentary, so that the mouth must always remain very wide open. I have failed to see how 

 this stomach can act as a digestive organ and have suggested that the canal system has 

 taken on the function of a stomach. This may also apply to other Aequoridae and account 

 for the large number of radial canals and the excretory pores on the circular canal. The 

 Maldive specimens are exactly like a figure of a medusa given by Forskal (1771), afterwards 

 called Mesonema pensile. I cannot find any evidence that this medusa has been taken since 

 Forskal's time. The Aequoridae require a thorough revision, but this can only be properly 

 done with the aid of a sufficient number of specimens as most of the species now have imperfect 

 descriptions. Special attention should be paid to the shape of the basal bulbs of the tentacles. 

 So far I have found the shape to be different in every species which I have examined. 



G. II. 93 



