THE LIVING WORLD. 
59 
to rupture. The scales are formed by concentric layers of a substance resem¬ 
bling neither bone, horn nor membrane, yet partaking somewhat of the nature of 
these three. It is by these that Agassiz established his classification of fishes, 
and so perfect was his knowledge in this respect that he had only to see a scale to 
determine the fish to which it belonged, though he may never have seen the species. 
The organs of touch in fishes are either the lips or antennae, and are very 
acute, as is that of sight, but the auditory nerves are rudimentary, so that their 
hearing is obtuse, if, indeed, they have this sense at all. They are marvellously 
susceptible to vibrations, as is evidenced by the excitement they exhibit at the 
shock produced by a person stamping upon the ground. So they have been 
trained to respond to the ringing of a bell, but the vibration, acting upon super¬ 
sensitive nerves in the gills, may be the effect, rather than sound waves striking 
upon auditory nerves. The sense of smell certainly exists, but experiments indi¬ 
cate that it is not acute. 
The sex of fishes may be determined by shape and color, but with the 
exception of sharks, rays, and a few others, there is no external difference 
between the two. The manner of propagation is not by contact, as in other 
vertebrates, but by a singular process which is yet imperfectly understood. 
Both male and female are provided with lobes lying along the intestinal canal, 
in which are contained what is called roe . At the breeding season the female 
voids the roe , which, on examination, are seen to be millions of very minute 
eggs, and these the male impregnates by voiding the roe which he carries, but 
which has now assumed a fluid state, somewhat resembling milk. It is not 
necessary that the sexes should be near each other at this time, as it has been 
shown by the fish culturists that eggs may be conveyed to any distance and 
then impregnated by squeezing the milt , as it is called, of the male upon them. 
Some fishes are viviparous, retaining the eggs within the oviduct until the 
young are sufficiently developed to appear. How impregnation is accomplished 
in such cases is a problem naturalists have not been able to solve, though it 
is probably by the female absorbing the milt of the male .through the vent. 
Generally, fishes are predaceous in their habits, being so voracious that 
they turn cannibal with the most natural instinct, and feed off the young of 
all kinds, not even respecting their own. In this, however, we see a wise pro¬ 
vision, since but for this check upon their multiplication the waters would soon 
become choked by their bodies, as no other creatures increase so rapidly, 
except it be a few insects. 
Of all creatures, fishes are most difficult to classify, because characteristics 
of form, structure and organization are so blended as to appear peculiar to 
hardly a single species. Every attempt thus far made, even by Agassiz him¬ 
self, has proven so unsatisfactory that I will not undertake _ a work which 
others much more competent than myself have given over as impossible, but 
will strive to increase the general interest in fishes by presenting them as 
they seem most naturally to occur in the order of progression. 
NEST-BUILDING FISHES. 
It has only been within comparatively recent years that the discovery was 
made that certain fishes construct nests in which to rear their broods. The 
species first noticed as performing this singular labor were the Sticklebacks , 
which are quite common in English aquaria. For a long while it was sup- 
