XXX 
THE VOYAGE OF H.M.S. CHALLENGER. 
In many Trachomedusse, and in almost all Narcomedusse, the marginal tentacles 
present the remarkable phenomenon of having migrated upwards on the dorsal surface 
of the umbrella from their original points of attachment close to the margin, so that they 
finally spring from a zone at some distance from the umbrella margin, with their roots 
plunged into the substance of the umbrella. In such cases accumulations of thread-cells 
form meridional urticating streaks (“ umbrella clasps ” or “ peronia ” of Haeckel) by 
which the bases of the tentacles continue to be connected with the marginal urticating 
ring. 
Comparison of the Medusa with the Hydranth. — The Craspedote Medusa as seen in 
the planoblast, however different its form may appear from that of the Hydropolyp as 
seen in Hydra or in the hydranth of the compound trophosome, may nevertheless be 
easily reduced to this fundamental type form. 
If we suppose the body of Hydra to be extended laterally in a plane immediately 
behind the hypostome, and on a level with the tentacular verticil, carrying between its 
upper and lower surfaces an extension of the body cavity, a hollow disc will be thus 
formed, having the tentacles springing from its margin, and with the hypostome which 
carries the mouth projecting from its centre. If now its upper and lower walls 
coalesce with one another throughout their entire extent, except along four or more 
radial lines running from the gastral cavity to the tentacular margin, there will result a 
disc traversed by radiating canals and margined by tentacles. This disc will manifestly 
represent the umbrella of a Medusa with its radial canals and marginal tentacles, while 
the hypostome will correspond to the manubrium ; and it only needs the distal ends 
of the radiating canals to become united to one another by the formation of a circular 
canal, and the proximal stem-like extension of the Hydi'a with its disc of attachment 
to be suppressed in order to convert the polyp form in all essential points into the 
Medusa form. The Medusa is thus nothing but a hydranth whose body-walls are 
extended in the form of an umbrella with offsets from the gastral cavity, and act as 
an organ of natation. 
Form and Structure of the Hedrioblast. — The hedrioblasts or gonophores, which 
never become detached from the trophosome as free-swimming zooids, are in the form of 
fixed sac-like buds. Of these two types must be distinguished. In one the medusoid 
conformation, however degraded, is yet sufficiently expressed to admit of its being 
recognised, the essential parts of the umbrella, including the endoderm lamella or 
“ vascular lamella,” being always present even though no gastro vascular canals may have 
been formed in it. In the other neither endoderm lamella nor gastrovascular canals 
have been formed, and the umbrella is represented solely by its outer ectodermal layer, 
which here lies directly on the spadix or endodermal cut de sac which forms the hollow 
axis of the gonophore. The first of these varieties of the hedrioblast is known as the 
medusoid , the second as the sporosac. 
