Hyalonema, the “glass-rope” sponge of Japan, consists of a bundle of from 200 to 300 threads of transparent silica, glistening with a satiny lustre like the most brilliant spun glass; each thread is about eighteen inches long, in the middle the thickness of a knitting-needle, and gradually tapering towards either end to a fine point; the whole bundle coiled like a strand of rope into a lengthened spiral, the threads of the middle and lower portions remaining compactly coiled by a permanent twist of the individual threads; the upper portions of the coil frayed out, so that the glassy threads stand separate from each other. The spicules on the outside of the coil stretch its entire length, each taking about two and a half turns of the spiral. One of these long needles is about one-third of a line in diameter in the centre, gradually tapering towards either end. The spirally-twisted portion of the needle occupies rather more than the middle half of its entire length. In the lower portion of the coil, which is embedded in the sponge, the spicule becomes straight, and tapers down to an extreme tenuity, ultimately becoming so fine that it is scarcely possible to trace it to its termination.
Within the mesoderm, and in oscule, was noticed a deep brownish-orange coloured shrunken membrane; this was traced to a parasitic polyp. Since this was first observed on an early specimen of the Japanese glass-sponge, the same parasite has always been found growing on and in all these curious sponges. The surface of the stalk above the portion embedded in the mud is seen to be covered with a warty crust of parasitic polyps. All the specimens of Hyalonema in the European museums in 1860 had their stalks overgrown with Palythoa, while many had their bodies also covered with another parasite, and which, fortunately for the sponge, did not form a sandy crust. The polyps, having no skeleton, dry up entirely, and leave behind no trace except the stain first referred to. Unlike a parasite, however, the polyps do not feed upon the juices and soft parts of the sponge, nor indeed do they share its food, but simply settle upon the sponge and feed upon any food that may chance to come within their reach.
The dredgings of the Challenger brought to the surface many entirely new forms of glass-sponges and from great depths. One of the most beautiful, known as Carpenter’s glass-sponge (Pheronema), is composed of concentric laminæ of silica deposited around a fine central axial canal. These form a gauze-like network throughout, but with no regularity of structure.
Clionæ.—Not the least wonderful circumstance connected with the history of sponges is the power possessed by certain species of boring into substances, the hardness of which might be considered as a sufficient protection against such apparently contemptible foes. Shells (both living and dead), coral, and even solid rocks are attacked by these humble destroyers, gradually broken up, and, no doubt, finally reduced to such a state as to render substances which would otherwise remain dead and useless in the economy of nature available for the supply of the necessities of other living creatures.
These boring sponges constitute the genus Cliona of Dr. Grant. They are branched in form, or consist of lobes united by delicate stems, and after having buried themselves in shells or other calcareous objects, preserve their communication with the water by means of perforations in the outer wall of the shell. The mechanism by which a creature of so low a type of organisation contrives to produce effects so remarkable is still doubtful, from the great difficulties which lie in the way of coming to any satisfactory conclusions upon the habits of an animal that works so completely in the dark as the Cliona celata. Mr. Hancock, in his valuable memoir upon the boring sponges, attributes their excavating power to the presence of the multitude of minute silicious crystalline particles adhering to the surface of the sponge; these he supposes are set in motion by ciliary action. In whatever way this action may be produced, however, there can be no doubt that these sponges are constantly and silently effecting the disintegration of submarine calcareous bodies—the shelly coverings, it may be, of animals far higher in organisation, and in many instances they prove themselves formidable enemies even to living molluscs, by boring completely through the shell. In this case the animal whose domicile it so unceremoniously invades has no alternative but to raise a wall of new shelly matter between himself and his unwelcome guest, and in this manner generally succeeds in barring him out.
From a close examination of the structural and developmental characters of the Spongideæ, it must be conceded that they belong rather to the flagellata Protozoa than to any other order. This was the view held by the late Professor Clark, and Mr. Saville Kent quite concurs in it.[69] Summing up the entire evidence adduced, scarcely a shadow of doubt is admissible concerning the intimate relationship that subsists between the Choano-flagellata and other flagellate Protozoa and that of sponges. The primary and essential element of the apparently complex sponge stock is the assemblage of collared flagellate zooids that inhabit its interstitial cavities under various plans of distribution. Individually these collared zooids correspond structurally and functionally in every detail with the collared units of such genera as Codosiga, Salpingœca, and Proto-spongia. The collar in either case presents the same structure and functions, exhibits the same circulatory currents or cyclosis, and acts in the same way for the capture of food. The body contains an identical centrally located spheroidal nucleus or endoplast, and a corresponding series of rhythmically pulsating contractile vesicles. The developmental reproductive phenomena are also strictly parallel. Both originate as simple Amœba or simple flagellate Monads, exhibiting no trace in their earliest stage of the subsequently acquired characteristic collar. Both again after a time withdraw their collar and flagellum, and assume the amœboid state; then, coalescing, enter upon a quiescent or encysted condition, and break up into a number of sporular bodies, and thus provide for the further existence and distribution of the species. The whole process again is much akin to that which obtains in the protophytic type, Volvox globator, which liberates from its interior free swimming gemmules that take the form of spherical aggregation of biflagellate daughter-cells. In their isolated state, on the other hand, the swarm gemmules of the sponge stock are directly comparable with the free swimming subspheroidal colony stock of the flagellate infusoria Synura, Syncrypta, and Uroglena, or with the attached subspheroidal clusters of Codosiga and Anthophysa.
Echinodermata, Hydrozoa, Polyzoa, Helminthoida.
Tuffen West, del. Edmund Evans.
Plate IV.