168 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
that the eyo of Salpa is derived from a portion of the nervous system 
which is not represented in Vertebrates. 
Optic Organ of Salpa.* — Dr. E. Goppert has investigated the optic 
organ in five species of Salpa, — S. africana-maxima Forsk., S. scutigera- 
confcederata Cuv.-Forsk., S. runcinata-fusiformis Cham.-Cuv., S. (Cyclo- 
salpa ) pinnata Forsk., S. democratica-mucronata Forsk. The eye most 
resembles that of Ascidians and Pyrosoma, but is decidedly different 
from either, and is homologous with an ordinary Vertebrate eye only in 
the wide sense that it arises from part of the central nervous system. 
In the solitary forms the organ arises in a horse-shoe-shaped dorsal 
portion of the ganglion, consisting of central fibrous substance and a 
peripheral layer of cells. Some of the marginal cells become optic 
cells — large, polyhedral, club-shaped, or pyramidal elements, with the 
thicker nucleated end towards the light. In one case there were spindle- 
shaped cells between the club-shaped cells. In most cases the optic 
cells contain “ phaeospheres,” and peculiar thickenings of the cell-walls, 
both probably with the function of rods. The sensitive elements occur 
in groups ; the largest group is horse-shoe-shaped, and associated with 
a pigment-layer which excludes the rays of light, except in definite 
directions. In S. democratica-mucronata the nerve-fibres unite with 
the ends of the sensitive elements which are towards the light, but 
in S. pinnata the reverse is the case. This difference is, however, 
explained by structural peculiarities. In all but S. democratica-mucronata 
there is an eye-chamber formed by a shield of epidermis ; this is of 
nutritive, but perhaps also of optical significance. The organ is not 
such as renders the forming of an image possible ; it is, however, well 
adapted for the localization of rays of light. 
In the social forms the optic organ is stalked, and protrudes obliquely 
forwards and downwards. Groups of cortical cells form retinae and are 
provided with a pigment-layer, though, except in S. democratica-mucro- 
nata there are also groups without pigment. In general features the 
sensitive cells resemble those in the solitary forms, but there is great 
variety in the disposition of the groups. In three cases the optic cells 
of the posterior part of the retina have their ends towards the light in 
connection with the nerve-fibrils, while in the anterior part the ends 
away from the light are so connected. The solitary forms “ see ” for 
the most part only dorsally, while the social forms can “ see ” ventrally 
as well. The advantage of this is explained. The optic organ is chiefly 
important in the orientation of the animals, e. g. as regards their distance 
from the surface of the water. 
/3. Bryozoa. 
Embryonic Fission in Cyclostomatous Polyzoa.f — Mr. S. F. Harmer 
gives an account of a case of embryonic fission in Crisia ramosa , found 
at Plymouth, which appears to be without parallel in the Animal 
Kingdom. His general results are as follows : — 
(1) The ovicell, which is morphologically equivalent to a zocecium, 
developes in the same way as an ordinary zooecium. 
* Morphol. Jahrb., xix. (1892) pp. 250-94 (3 pis., 1 fig.), 
t Quart. Journ. Micr. Sci., xxxiv. (1892) pp. 199-241 (3 pig.). 
