188 PERCY SLADEN TRUST EXPEDITION 



23. Mesonema pensile (Modeer) 1791. 



Mesonema pensile, Browne, 1904, p. 733, pi. 55, fig. 4, pi. 57, figs. 2 — 9. 

 Mesonema pensile, Browne, 1905, p. 147, pi. 2, figs. 11 — 15. 

 Mesonema pensile, Maas, 1905, p. 42, Taf. 8, fig. 52. 

 Mesonema pensile, Maas, 1909, p. 26. 



Locality. Chagos Archipelago, Diego Garcia. 10 — ^14 fms. 10 July, 1905. 

 1 specimen. 



The genus Mesonema of Eschscholtz has undergone several changes in its definition. 

 In 1904 I defined it as follows : — " .^quoridse with numerous, simple, unbranched radial 

 canals. Stomach circular, with lower wall quite rudimentary. Mouth nearly as large as 

 the diameter of the stomach and cannot be closed." The type species of the genus is 

 Mesonema pensile (Modeer) 1791, and specimens similar to the type were described and 

 figured by me in 1904. Both Mayer (1910) and Bigelow (1909) have raised objections 

 to my distinguishing marks of the genus, and by which I separated it from JEquorea. 

 They rightly say that owing to the great contractibility of the lower wall of the stomach 

 it is difficult to assign to the correct genus certain species of the ^quoridae, and therefore 

 they consider it is advisable to recognise JEquorea only, and place Mesonema in its list of 

 synonyms. I clearly foresaw this difficulty, but had hopes that further researches on 

 the ^quoridae might lead to the permanent retention of Mesonema by adding other 

 characters so as to definitely separate it from j^quorea. 



Vanhoft'en (1911) also rejects my definition of Mesonema and brings forward a new 

 definition as follows : — 



Mesonema. Leptomedusae with numerous radial canals, and with large tentacles 

 considerably less in number than the canals. Tentacles with triangular and especially 

 heart-shaped bulbs. 



jEquorea. Leptomedusae with numerous radial canals, with fully developed 

 tentacles about as many or more than radial canals. Tentacles with conical basal 

 bulbs. 



Vanhoffen's characters for separating the two genera may be an improvement on 

 mine. It is quite easy to isolate the extremes as separating characters, whether one uses 

 the size of the mouth, the number of canals in relation to the number of tentacles, or the 

 shape of the basal bulbs, but the difficulty is to assign to the right genus those specimens 

 which come near the border line. We have a very imperfect knowledge of the species 

 belonging to the genus jEquorea, using the term in its widest sense. Some of the 

 species exist on paper only, as their descriptions are too vague for any further use, a 

 few have been fully described and figured, and many disputed as to their validity 

 by systematists. 



Vanhofien (1911) considers that ^Equorea macrodactyla, Brandt, JEquorea maldi- 

 vensis, Browne, and JEquorea parva, Browne, are all identical with Mesonema coelum 

 pensile of Modeer. It is still, however, my firm opinion that JHquorea macrodactyla and 

 Mesonema pensile are quite distinct species. Although the original descriptions of these 

 two old species are rather vague and their figures more like rough sketches, still their 

 names are now associated with medusae which have been described and figured according 



