42 HOWARD AYERS. 
motions. The rotating motion resembles that of the spines in 
every respect; the jerking motion consists of sudden bendings 
from side to side, and may be observed when the animal is 
irritated or the organs stimulated, as by the addition of a drop 
of acetic acid to the seawater in which the Urchin is placed or 
by raising the saltness of the seawater. If removed from the 
Urchin in connection with a small part of the test they will 
remain alive in seawater for several days. 
As their name indicates these organs are spheroidal in shape, 
though varying in form in different species, and not unfre- 
quently in the same individual; thus in Spatangus the normal 
form seems to be that of an almost perfect sphere, though the 
elongate form is sometimes found. For Strongylocen- 
trotus droebachiensis the normal form is egg-shape; the 
spherical form is very seldom seen. There is a typical form 
for each species, i.e. a particular fashion of the curves of the 
head and neck common to the majority of the globules of any 
individual. In each spheerid the following external parts (fig. 
20) are readily distinguishable :—The base, which is com- 
posed of a mamelon of the test not different from those of the 
small spines. The joint, that part which forms the connec- 
tion between the base and globule, and which is composed for 
the most part of muscle-cells and fibrous tissue in the form of 
a band surrounding the ball and socket of the joint. The 
globule or body of the spherid, which is itself composed of 
the neck and the head. This method of naming the parts is 
natural, and is necessary to the proper understanding of the 
structure of the organ. 
From a study of the structure of the calcareous body of the 
spherida, Lovén concluded that each globule was composed of 
two sorts of calcareous matter of different physical properties. 
That forming the larger part of the body he named the 
“vitreous calcareous substance,” and the branching thread- 
like network he called the “ reticular calcareous tissue.” As 
regards this latter, however, his interpretations are faulty, as 
the experiments which I will give further on conclusively 
prove. 
