ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
309 
covery lies in the circumstance that of existing animals the Plirynid® 
appear to be most nearly allied to the immediate ancestor of the 
Spiders, and we may conclude, therefore, that originally the silk in 
spiders was utilised for the purpose of making a case for the eggs. The 
simplest stage now to be observed is probably to be found among the 
females of the Drassid®, some of whom spin a temporary retreat for 
themselves and their young at the breeding season. Others utilise the 
retreat as a permanent dwelling-place. If we conclude that a simple 
tube was the primitive form of nest, we may see that the evolution of 
web-spinning has been carried out along two main lines. Along one 
there is a gradual elaboration of the tube until it ends in the complex 
trap-door nest. Along the other the tubular nest either disappears or 
remains simple, while it is superposed by a new structure, the net for 
ensnaring prey. In the latter part of his essay Mr. Pocock discusses 
the use of radiating threads, and concludes that in constructing the net 
of radiating and concentric threads the Epeira economises both time 
and silk, and renders her snare as strong and as serviceable, and yet as 
delicate and invisible as possible. 
€. Crustacea. 
Post-oesophageal Herve-eord of Crustacea.* — Mr. W. B. Hardy has 
a memoir on some histological features and physiological properties of 
this portion of the nerve-cord. With regard to the post-oesophageal 
nerve-cord in Branchipus, he points out that the central nervous system, 
taken as a whole, consists of two cords of nerve tissue, which run the 
length of the body and are connected anterior to the mouth by the brain, 
in the region of the mouth by the circumoral ganglion, and posterior to 
the mouth by the transverse commissures which connect the various gan- 
glionic enlargements. Each nerve-cord consists of a mass of fine nerve- 
fibres invested more or less completely by nerve-cells. The ganglionic 
regions differ from the inter- ganglionic chiefly in the development of a 
very fine plexus on the ventral aspect of the cords. The ganglionic 
regions are also characterised by the large number of nerve-cells, but 
the inter-ganglionic and ganglionic regions are not sharply defined from 
one another. A description is given of the connections of the nerve-cells, 
and of the distribution of the elements in a typical ganglion. In the 
crayfish three pairs of nerves are found to arise from each of the first 
five abdominal ganglia. The anterior pair, which rise directly from the 
ganglion, contain a large number of fine or afferent fibres, and com- 
paratively few large or efferent fibres ; these supply the appendages 
with motor and sensory fibres, and also the skin of the sternum and 
pleura. The posterior ventral nerves contain relatively more large 
fibres ; these supply the dorsally placed extensor muscles and the 
dorsal skin. In the third segment, or segment next following, the 
posterior dorsal nerves are purely motor, and innervate the flexor 
muscles. The author also discusses the general relations and structure 
of the second abdominal ganglion, as well as the arrangement of its 
nervous elements. The internal connections of the anterior and the 
posterior pairs of nerves are described, as well as those of the posterior 
* Phil. Trans., clxxxv. B. (1894) pp. 83-117 (4 pis.). 
