22 
STRUCTURE AND LIFE-HISTORY OF A SPONGE. 
we shall gain a fair idea of this arrangement. Further, it will be 
seen that however close the test-tubes lie to one another, narrow 
three-sided canals will remain between them, one such canal 
between every three mutually adjacent tubes. Precisely similar 
canals are left between the tubes of the sponge, and are known as 
inter-canals, whilst the tubes themselves are termed radial tubes. 
The radial tubes have not continuous walls like those of a test- 
tube, but are perforated all over by a number of minute apertures, 
or pores. Those pores which occur over the projecting conical 
ends of the tubes open immediately to the surrounding water ; 
those which occur along the sides of the tubes, where they are not 
in contact, open into inter-canals, and so indirectly into the outer 
water, while those finally, which occur along the line of union of 
the radial tubes serve as a means of communication between 
these tubes, and do not open into the outer water at all, except, of 
course, by way of the stomach through the mouth. 
Histology . — The proper wall of the stomach, and alike of the 
radial tubes, consists of three layers of tissue, an outer, or ectoderm j 
an inner, or entoderm ; and a middle, or mesoderm. 
The ectoderm., which covers the stomach and the exterior of the 
radial tube lining the inter-canals, consists of a single layer of plate- 
like polygonal cells, o'oi5 to 0*025 mm. in diameter. Each 
contains a circular cake-like nucleus, bounded by a nuclear 
membrane, and full of watery fluid, in which are suspended two 
or three very refractile granules, or nucleoli ; the protoplasm in the 
centre of the cell, surroundiug the nucleus, is more or less granular, 
but towards the margin perfectly clear. 
The mesoderm, which succeeds, is of a very different nature, the 
great mass of it consists of a clear transparent jelly-like material, 
which does not stain with carmine or other colouring re-agents : 
dispersed through this jelly-like matrix are numerous cells of 
branching form. Each contains in the middle a long oval 
nucleus, with spherical nucleolus, and is surrounded by finely 
