526 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
pharyngeal glands present some very remarkable characteristics. Below 
the brain in the region of the month transverse sections show a peculiar 
tissue, the appearance of which directly attracts attention. This tissue 
is formed of cells of considerable size arranged in small groups of from 
three to five, and connected with one another by a fundamental connective 
tissue. These elements have an appearance which is not unlike that of 
the Noctilucse. One immediately recognises a kind of hilum whence 
richly branched prolongations radiate in all directions. It is from this 
hilum that the excretory duct of the glands arises. The canal swells 
into a vesicle, into which there open by a very small number of pores 
the proximal ends of the radiating canaliculi. The small number of 
these orifices is due to the fact that the canaliculi open into large lacunar 
spaces. The radiating canals branch more or less largely, but they 
only extend into the clear region of the protoplasm of the cell. This 
protoplasm exhibits an alveolar structure. The nucleus, which is 
eccentric in position, is surrounded by a clear zone which varies in 
appearance and constitution. 
Retina and Optic Ganglia in Decapoda.* — Mr. G. H. Parker, who 
has specially studied the eye of Astacus , gives a detailed account of his 
results. Beginning with a section on the general structure of optic 
stalks, he points out that the nervous organs contained within these are 
so complex that it is well first to consider the shape and position of the 
stalks themselves. The nervous structures contained within them have 
received such a variety of names that an accepted nomenclature for them 
can hardly be said to exist. In dealing with the retina Mr. Parker 
commences by a consideration of its form. This is by no means easy, 
as the extent of the retina is not only unequal in different directions, 
but the curvature of its outer surface also varies. Dealing next with the 
ommatidia he points out, as is no doubt well known, that the results of 
investigators are by no means in full agreement, and he ventures there- 
fore to redescribe these organs, dealing at length however with those 
points only where a difference of opinion exists, or where he has gained 
a clearer insight into their structure. The migration of retinal pigment 
is next considered, and it is stated that the black pigment of the distal 
and proximal retinular cells is a means of controlling the amount and 
quality of the light that reaches the rhabdomes. The author’s critical 
investigation of the various theories of vision results in showing that 
when the rhabdome is surrounded by pigment its whole length can be 
penetrated only by very strong light, and, secondly, they confirm the 
belief entertained by Muller, Grenacher, and Exner that the image in 
the compound eye is a single upright one for the whole retina, whose 
perceptive elements, the rhabdomes, receive each a single impression. 
This result, however, is so contrary to that obtained by many investi- 
gators that the author pursued his inquiry, and observations that he 
made left no doubt in his mind that when the retinal pigment in Astacus 
was adjusted for very dim light the image formed by the dioptric 
apparatus was a superposition one. Mr. Parker’s account of the optic 
ganglia commences with some notes on their position, after which the 
topography of the ganglia is discussed in detail. As to the optic nerve, 
* MT. Stat. Zool. Neapel, xii. (1895) pp. 1-73 (3 pis.). 
