8 ZOOLOGICAL EXERCISES. 
animals, we find that an organ, similar at first in both, 
develops differently, we call these organs homologous; they 
have a similar fundamental structure. Such are the fore 
limbs of a bird and a mammal. When we find that in the 
development of an animal, two parts, placed differently 
with reference to the axis of the body, originate similarly, 
these organs are said to be homodynamous, whether they 
develop similarly or not; such are the fore and hind limbs — 
of a vertebrate, or the appendages of an arthropod. When 
two organs of two different animals originate in different 
ways, and yet have the same function, they are said to be 
analogous; such are the gills of a fish and of a crab. 
Analogy shows a merely superficial resemblance, and is of 
little importance in Zoology ; but homology shows a funda- 
mental resemblance, and is of the greatest importance in 
tracing out the relationship between animals. 
Another special department of Morphology is Histology, 
which is limited to the study of the minute structure of the 
tissues. The principal tissues are (1) Connective tissue, the 
substance which connects the different organs together, and 
also separates them one from the other. At first it is a 
cluster of cells with thin walls, and is then known as 
indifferent tissue, as it may develop into any of the follow- 
ing varieties of connective tissue. Cellular tissue, consisting 
of rounded cells. <Aveolar tissue, consisting of a matrix 
with irregularly disposed fibres, leaving wide spaces 
between them. White fibrous tissue, in which the fibres 
are close together, and arranged in bundles, which may or 
may not be parallel; the fibres in both this and areolar 
tissue swell up and become indistinct on the addition of 
weak acetic acid, Elastic tissue, consisting of fibres of a 
yellow colour, which are not affected by acetic acid: 
Cartilage, consisting of more or less rounded cells imbedded 
