OF THE HYDROMEDUS^. 387 



certain points which are now known to be shared by other genera, and others which 

 are only of specific importance. Haeekel's diagnosis (31, p. 06) is based in part upon 

 an erroneous interpretation of McCrady's account of our species, and I therefore give a 

 new statement of the distinctive characteristics of the genus. 



Genus-Diagnosis. Tiarid with numerous tentacles in a single row, and a single ocel- 

 lus on the inner or axial side of the bulb of each. No gastric peduncle from the gelat- 

 inous substance of the umbrella, from which the digestive cavity is suspended by a carti- 

 lage-like mass, made up of the greatly enlarged endoderm cells of the radiating tubes. 

 ~No mesenteries. Four simple perradial reproductive organs in the walls of the diges- 

 tive cavity, separated by deep furrows with smooth surfaces. < )ral lips fringed with stalked 

 bunches of lasso-cells. 



The stalk which suspends the digestive cavity of Turritopsis from the centre of the 

 sub-umbrella is not a gelatinous prolongation from the umbrella, but a peculiar struct- 

 ure, made up of the greatly enlarged endoderm cells of the radiating tubes, which, in the 

 adult, are pendent from the sub-umbrella, as in Haeekel's figures of Callitiara ( 3, PI. 3) 

 but so greatly thickened as to form a solid cartilage-like mass, through which the four 

 small channels pass down to the digestive cavity, into which the chorda-cells also extend. 



McCrady's figures and minute description of this structure are so very clear that there 

 should be no room for mistake. He says (48, page 3) "The stomach surrounded by the 

 ovaries occupies the lower half (of the peduncle), but above is a mass of very large cells 

 filled with a clear substance like that in the upper part of the disk in Oceania. This 

 portion is traversed by the four ascending chymiferous tubes, around which the large 

 cells are arranged with much regularity, and which, on reaching the muscular disk, arch 

 over it to descend through its substance as vertical tubes." On p. 5, he says, "Return- 

 ing now to the vertical tubes, we find that before entering the tissues of the bell, they 

 traverse the clear portion of the proboscis. Here they do not preserve the even, some- 

 what flattened form which they have in the disk, but assume a rather irregular outline. 

 This appears to be due to the circumstance that the canal occupies the somewhat irreg- 

 ular cavity left between the juxtaposed ends of the large cells composing the transparent 

 part of the proboscis. How these cells are arranged radiately is shown in a diagrammatic 

 cross-section at tig. 7. A small quadrangular space is left between the four masses thus 

 formed, which is, probably, tilled with the same clear substance which tills the cells. The 

 tissue SO formed is not confined to the tubes, though it has there its greatest development; 

 it spreads also downward over the several lobes, bul in this portion the cells are very much 

 smaller. Around the tubes the cells are of a somewhat pyramidal form, their bases turned 

 outwards, the apices inwards, to meet the chymiferous canal." 



Keferstein (3(5, p. 26) correctly describes the peduncle of his Oceania polycirrha, \\ hich 

 is a true Turritopsis, as "made up of large transparent cells which look like a network;" 

 but Haeckel (31, p. 66) misled, no doubt, by the close resemblance between Turritopsis and 

 Callitiara, has described the structure as an ordinary gelatinous gastric stalk, although 

 it is, in reality, a very different structure from the peduncle of Eutiina or that of the 

 ( J-eryonidse. 



McCrady failed to discover that the cells are nothing more than the greatly thickened 

 walls of the radiating tubes, but in other particulars his account is very accurate, al- 



