129 
of Edinburgh, Session 1875-76. 
As soon as the powerful hooks of the fifth segment appear, the 
entire area — from the alimentary canal to the body wall — is occu- 
pied by their muscular apparatus. This consists of a dense series 
of fibres, which slant from the matrix of the bristles superiorly 
upward and outward to decussate with the fibres at the upper and 
outer angle of the body-wall. A still stronger series of fibres occur 
in the inferior division ; the inner are nearly vertical, the rest 
incline downward and outward. It would appear, therefore, that 
this powerful muscular mass chiefly acts on the hooks, so as to 
bring their curved points against the wall of the tube or tunnel. 
The strong inferior fibres likewise gain additional purchase by 
passing through the ventral longitudinal layer to be attached to 
the basement-tissue of the hypoderm. In this region the nerve- 
cords form two almond-shaped bodies in transverse section in the 
ventral hypoderm, and they are separated by a distinct interval. 
Posteriorly the nerve-cords still remain separate, and a large 
neural canal lies between them. A well-marked pennate process of 
the ventral longitudinal muscle occurs at its inner (median) edge. 
The foregoing forms, in conclusion, were compared with the 
structure of Mcea mirabilis , Johnst., an aberrant member of the 
Spionidas. 
2. Note on Circular Crystals. By E. W. Dallas. 
At long intervals notices of circular crystals have appeared before 
this Society. In 1853 Sir David Brewster read a paper on the 
subject, which followed one by Mr Fox Talbot in 1836, and which 
again had been preceded by one from Sir David Brewster about twenty 
years before. It is not easy to account for these long intervals, 
unless they may be attributed to difficulty and uncertainty in 
manipulation, for except in very few instances the crystals observed 
by Sir David Brewster are of microscopic size, and, he remarks, require 
the perfection of optical appliances for their observation, and natu- 
rally so when crystals of the 200th of an inch in diameter are looked 
upon as of respectable size. 
Some time ago, being occupied with the subject, I found that by 
impeding crystallisation by means of gum arabic, circular crystals 
were formed of a greater size.* This took place with certain salts, 
* Perfect crystals were exhibited up to two inches in diameter. 
VOL. IX. 
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