the market, the
skeleton is almost entirely composed of fibers of pure "spongin." These
fibers are so close together as to draw up water by capillary action,
and, indeed, a great deal in the value of a sponge depends upon the
fineness and tenuity of these fibers.

Dr. Ledenfeld again illustrated this stage of his lecture by means of a
number of microscopic slides in which the variety of shape and size of
these spicules and "spongin" fibers were shown. The spicules are some
crutch-like, others spined or echinated, while the deep-sea sponges
appear to grow long thick spicules, which attach the sponge to the
ground by means of grapnel-like ends. In some cases the skeleton seems
to be more or less replaced by sand, the small grains of which are
cemented together by the "spongin."

Dr. Ledenfeld then drew attention to the presence of more highly
developed organs in the sponge. Muscles pervade the whole tissue of the
sponge, but are found more particularly in the superficial parts. One
set of muscles affect the size of the inhalent pores, causing them to
contract or expand, while another set are able to close the pores
altogether, thus acting as a protection from the attack of an enemy. All
these muscles are composed of spindle shaped cells, which are capable of
spasmodic motion, but recently in an Australian sponge, the _Euspongia
canalicula_, the lecturer said he had observed muscles approaching very
nearly in character those of the human frame.

That sponges have nerves is a discovery of recent date by a member of
the Royal Microscopical Society. Dr. Ledenfeld also about the same time
found indications of the presence of a nervous system, but the form in
which he observed the nerves at first apparently differed from those
observed simultaneously. This difference, however, he afterward found to
be due to the manner in which the section had been prepared for
observation. The nerves consist of two cells at the base of a cone-like
projection on the epidermis, and from each cell a fiber runs to the
point of the cone, besides several others connecting them with the
interior of the sponge.

It is remarkable that here again Aristotle has predicted that sponges
have a nervous system, basing his statement on the fact that ancient
Greek mariners foretold storms by the alleged contraction of the sponge.
The reproductive organs of sponges are also very highly developed, and
both ova and spermatozoa are found throughout the sponge,