HYDROZOA. 
Coil. 12 
especially not having been made use of. It is impossible here to give a 
fuller review of the light thrown by these investigations upon the modi- 
fications of the different organs and tissues, according to their degree of 
evolution or differentiation. 
The larger work of the two Hertwigs (13) is devoted to the histological 
analysis of the nervous tissue and sense organs, and to the discussion 
of the conclusions respecting the genesis of these organs in the animal 
kingdom, which may be deduced from the observations. It is based upon 
the examination of seventeen genera of Mediterranean Medusce. In the 
craspedote MeduscR there exists a continuous central nervous system, con- 
sisting of a double marginal ring, divided by the basement membrane of 
the velum ; both consist of delicate fibrils and ganglionic cells, the 
superior is chiefly, but not exclusively, a sensorial centre, the inferior, 
in like manner, chiefly a motorial ; the ciliated epithelial cells protecting 
the superior nerve ring, and communicating with it through their basal 
filaments are a special modification of the ectoderm (sense-cells) ; the 
auditory cells, and the cells of vision connected with or forming the special 
sense-organs are again specially modified sense-cells. Ganglionic cells 
are found scattered or forming a plexus, representing a ganglionic nervous 
system, on the sub-umbella; special nerves are rarely found, for instance 
the auditory nerve in Geryonia. Peculiar organs of touch are shown in 
the extremities of the inter-radial tentacles in Rhopalonema and the mar- 
ginal combs in this genus and in Aglaura. Eyes occur in the “ Ocel- 
latm” (^Anthornedus(B^ Hackel, viz., in those Medusce which are nursed by 
Tubularian Hydroids, and have the organs of generation placed in the 
walls of the stomach), always in the base of the tentacles, sometimes con- 
sisting of a pigment-spot (sense-cells enveloped by pigment-cells) alone, 
sometimes provided with a lens (a cuticular development). Ocelli may 
occur also in some other craspedote (^Tiaropsis^ for instance), but 
auditory organs never occur in the Tubillarian Medusoe. In those again, 
which are nursed by Campanularian Hydroids, viz., the “ Vesiculatm ” (or 
Leptomedusce, Hack., whose organs of generation are placed along the 
radial canals), the auditory organs (cells containing calcareous concre- 
tions, “ otoliths,” supported by auditory cells), are connected with the 
inferior nerve-ring and placed in hollows or in closed cysts, according to 
the genera. Auditory organs are only rarely connected with the ten- 
tacles ; their position is chiefly marginal, and they are found in many dif- 
ferent modifications and degrees of development. In the JEginidce^ they 
are naked “ear-clubs,’^ modified rudimentary tentacula, containing one 
or more otoliths in their axial cells, placed on special cushions and sur- 
rounded by specially developed auditory cilia ; in the young Rhopa- 
lonema^ they have the same character, but during the growth of the 
animal they are transformed into cysts, in which the stalked “ear-clubs” 
are inclosed, held in place by means of their auditory cilia. In the 
Geryonidm, the similarly constructed auditory organs are sunk in the jelly 
and provided with two auditory nerves. In the Acraspedca^ the central 
nervous system is interrupted, being confined to the bases of the marginal 
“ sense-bodies,” which may be regarded as modified tentacula, but with 
less definite physiological action. In some {Pelagia)^ they only contain 
