214 
E. A. SCUAFER. 
We also find that cells which are set aside for the sole per- 
formance of special functions invariably become modified in the 
arrangement of their living substance, and in some cases also 
in its chemical nature. The structural changes which thus 
accompany and indicate the assumption of any special function 
by cells, constitutes the science of histology. It would be 
carrying us too far from our immediate subject to enter into a 
detailed account of the nature of these structural changes here, 
so I wiU only point out two facts wdth regard to them. One is 
that the special modifications of^structure which the cells assume, 
that have been set aside for the sole performance of special 
functions, are similar throughout the animal kingdom in all 
essential features. The second is that in the lowest types 
where they occur the structural characters appear before the 
segregation of the cells is complete, whilst on the other hand 
in the development of an individual belonging to the higher 
types such segregation may have been long effected before either 
the functions or the accompanying structural changes in the cells 
begin to be manifest ; another instance of premature segregation. 
We have thus far been speaking of the separation of special 
sets of the cells of which a simple organism is composed for 
the performance of special duties. But in the united or com- 
pound organisms, before considered, ;t very frequently happens 
that one or more of the units of which the compound body is 
composed becomes altogether specially modified for the perform- 
ance of one duty to the exclusion of others. Thus we see that 
some of the individual buds, of which the compound organism 
of a hydroid polyp consists are adapted for purposes of prehen- 
sion and alimentation, whilst others are adapted solely for 
reproduction. This localisation of function in the different buds 
of a compound organism is carried to an extreme degree in the 
Siphonophora — animals which on the whole resemble the hydroid 
polyps, but instead of being fixed to a rock at the bottom of 
the sea, float near the surface of the water, looking like strings 
of beautifully iridescent hyaline beads. In them we find the 
specialisation of individuals to have proceeded so far that whilst 
one of the innumerable individuals which have been produced by 
budding from the original single one, is set aside to perform the 
purely mechanical purpose of suspending the united colony near 
the surface of the sea, and is changed into a minute balloon 
enclosing a bubble of air ; others are deputed to propel the 
organism through the water and become wholly transformed into 
so many pulsating bells ; others again have merely to receive 
tactile impressioiis from objects in the external medium, and are 
chiefly metamorphosed into long feelers ; others are occupied 
solely with the seizure and ingestion of victims for the food of 
