The sponge, like the tape-worm, has no stomach, but must absorb its food through the outer skin from matter in a soluble state, similarly to the roots of trees.  This process of absorption is probably accomplished in the interradial or ciliated chambers, more probably in the former, as the latter are generally considered excretory in function.  Lime or silica must also be absorbed from the water by most sponges in order to make up the skeleton.  The skeleton of calcareous sponges consists of a number of spicules composed of carbonate of lime.  These spicules are of very varied though regular shape, but ordinarily assume a rod-like needle shape or else a stellate form.  In silicious sponges the spicules are composed of silica, and are generally deposited around axial rods in concentric layers.  The spicules are joined together and cemented by a body that has been named “spongin,” which has much the same chemical composition as silk, and, like silk, is very elastic.  In some varieties of sponges, especially in the kinds which come into 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.