THE extreme difficulty of pointing out the exact limits of the division between animal and vegetable life in the lowest structures of organized bodies has already been alluded to. The error of former philosophers, who pretended that the boundaries which separated the plant from the animal were so well-defined that the distinction was rendered obvious, has become evident by the advance of science, and I believe that the idea of a chain of beings where all created substances pass by insensible degrees from the simplest to the most complicated forms, has been revived, and finds at present a more general adoption than twenty years ago.

We have already alluded to the doubts which exist whether the Diatomacea are of vegetable or animal origin; this however is not the only

instance in which near the limits of the great division of organized bodies our doubts are raised whether we see a plant or an animal before us; and this does not solely refer to microscopic animalcules, but applies likewise to more perceptible bodies; and as such I will instance the Sponges. Dr. Johnston was one of the first who adopted the opinion that the River Sponge (Spongilla fluviatilis) is a vegetable production, and the researches of Hogg and Ehrenberg remove all doubts that the marine sponges (Spongia marina) are of the same origin. Professor Ehrenberg observes that his numerous examinations of sponges from all seas and all geological formations where they had hitherto occurred, made him acquainted with hundreds of forms, doubtless of a regular structure and easy of recognition, but never with such forms as indicated development of organization. Sponges exhibit no signs of sensibility; nor did Mr. John Hogg during his investigations observe any trace of animal organization, or the least symptom of sensation, or any powers of contraction and dilatation in Spongilla fluviatilis. Being convinced by his researches of the vegetability of the River Sponge, he began to examine many of the Spongia marina, and found on comparison that there is scarcely even a generic difference between Spongilla and many of the Spongia. The vegetable nature of these substances is now very generally acknowledged among naturalists. Mr. Hogg, considering to what order of plants the freshwater and the sea sponges should be referred, proposed to classify them in a separate order, "Spongia," which ought to be placed between the order Fungi and that of the Alga1.

In Spongia, or as they are named in some systematic works Porifera, the cellular tissue of the body is permeated in all directions by anastomosing and ramifying ducts, which begin by minute pores closely distributed over every part, increasing in size, and leading to orifices on the surface from which water is seen to issue. In Spongia and Ephydatia the fibres are soft, and appear to consist of a jelly-like substance; in the genus Tethya they are stony and diverge from the centre to the circumference of the mass. The form of the Spongia is irregular or branched, that of the latter (Tethya) globular. They assume a great variety of shapes, resembling tubes, fans, shrubs, vases, &c. The following occur principally in the sea around Barbados:

Spongia tubæformis, Lam., No. 64. (Sloane, t. 24. f. 1.)

fistularis, Lam., No. 65.

vaginalis, Lam., No. 68. (Sloane, t. 24. f. 1.)

xerampelina, Lam., No. 90.

arborescens, Lam., No. 98.

1 Annals of Nat. History, vol. iii. p. 461.