89 INVERTEBRATE ANIMALS. 
siderable group of naked-eyed JZedus@ to which this explana- 
tion has not hitherto been shown to apply. In most of the 
members of this group the course of development is quite un- 
known, and therefore their true nature is a matter of doubt. 
Two families, however, of this group are stated to produce eggs 
which develop directly into eduse, such as those which gave 
origin to the eggs; and if this observation is confirmed, these, 
at any rate, must be regarded as true Meduside. In the 
meanwhile, therefore, it is best to regard the group of the Dzs- 
cophora or Meduside as of a questionable nature, and as in- 
cluding forms which may ultimately be shown to be nothing 
more than the detached zodids of other Aydrozoa. Under 
these circumstances it will not be requisite to do more than very 
briefly to describe the anatomical structure of a typical Weduszd; 
and this is the less necessary, since it will be seen at once that 
the structure is in all essential respects identical with what has 
been already described in speaking of the free medusiform 
gonophores of the Hydroid zoophytes. 
Fig. 25.—Morphology of Mcduside. a Medusid (Thausmantias) seen in protile, 
showing the central polypite,:the radiating and circular canals of the disc, the 
reproductive drgans borne in the course of the former, the marginal bodies and 
tentacles; 4 The same, viewed from below. The dotted line indicates the mar- 
gin of the ‘‘ veil.” 
In all the naked-eyed Meduse, of which Thamantias (fig. 
25) may be taken as a good example, the general structure 1s 
briefly as follows: The hydrosoma is perfectly free and is 
oceanic, being found swimming near the surface in the open 
ocean. The body is composed of a thick, transparent, gelatin- 
ous disc or swimming-bell (the nectocalyx), by the pulsations 
of which the animal is driven through the water. From the 
under surface or roof of this bell-shaped disc is suspended a 
single polypite (the manubrium), which bears to the disc the 
