PROGRESS OF MICROSCOPICAL SCIENCE. 
143 
the ganglia themselves are thick. Five ganglia were observed corre- 
sponding to five anterior pairs of members. It is probably not until 
after the first moult at least that the adult form of the nervous cord 
is attained. 
Some interesting questions in the morphology of Limulus arise 
in connection with this discovery of the original separation of the 
ganglia of the head, which I will simply touch upon. The brain of 
Limulus differs remarkably from that of the normal Crustacea, 
i. e. the Decapods, in sending off no antennal nerves, but only two 
pairs of optic nerves, there being in fact in Limulus no antennae. 
In the spiders and scorpions the disposition of the nervous system 
only resembles that of Limulus in the thoracic and cephalic ganglia 
being somewhat consolidated, but the brain is situated far above the 
plane of the thoracic mass, and the commissures are very long, and 
here also there are no antennal nerves, no antenna} being present, 
but a pair of nerves is distributed to the mandibles. The general 
analogy in the form of the anterior portion of the nervous cord to 
the Arachnidan, by no means proves satisfactorily to my mind that 
the Limulus and Merostomata generally are Arachnida, as some 
authors insist, for, besides the remarkable difference in the form and 
position of the supraoesopliageal ganglion above mentioned, there are 
other differences of much importance, which separate the Mero- 
stomata from both the Arachnida on the one hand, and the Crustacea 
on the other. 
It will now be a matter of interest to study the development of 
the nervous cord in the Arachnida, at the stage where the cephalo- 
thoracic ganglia are separate, and compare them with the same stage 
in Limulus. 
The result may possibly show that the appendages of the anterior 
region of Limulus are in fact cephalic appendages or mandibles and 
maxillfe or maxillipeds, and in part truly thoracic ; as in the spiders 
and scorpions the nerves to the maxilhe and legs are distributed from 
a common cephalothoracic mass of concentrated ganglia. 
The Siliceo-Fibrous Sponges. — A valuable paper has been read on 
this subject before the Zoological Society of London, by Dr. J, S. 
Bowerbank, F.R.S. It is styled, “A Monograph of the Siliceo-Fibrous 
Sponges,” part iii., being the third of a series of memoirs on this class 
of sponges. A second communication from Dr. Bowerbank to the 
Zoological Society contained the seventh part of his contributions to 
a General History of the Spongiadte. 
The Spermatozoa of the Petromyzon have been investigated by 
Mr. G. Gulliver, F.R.S. , who has recently read a paper on this subject 
before the Zoological Society of London. 
Structure of the Volcanic Dust of Barbadoes. — The structure of this 
dust, which fell in such quantities in the island of Barbadoes in 1812, 
has been examined by Professor Hull, F.R.S., who gives the following- 
account in the ‘ Geological Magazine’ (June, 1875) : With an objective 
power of fifty-five diameters, the dust is seen to consist of angular, or 
subangular grains of a translucent reticulated mineral, amongst which 
