290 
THE VOYAGE OF H.M.S. CHALLENGEE. 
The illustration (PL XXX. fig. 17) represents the spongin uniting two crossed 
spicules as granular — it is not really so ; and the margins of a spongin mass are usually 
more sharply marked than in the illustration. 
Development of the Desma . — The desmas commence within a large mother-cell as 
minute calthrops with cylindrical actines (PL XXX. figs. 20, 21), the surface of the spicule 
presents the eroded appearance characteristic of most very young forms of spicules ; its 
actines measure about 0‘016 mm. in length, and an axial fibre extends from their origin 
to their termination, so that the whole spicule is moulded on a tetrad axis. In this 
stage it resembles the young calthrops of Dercitus and its allies. The mother-cell, about 
0'028 mm. in diameter, consists of granular protoplasm, which stains deeply with reagents, 
and presents an oval nucleus, about 0’008 mm. in diameter, with a small, spherical 
nucleolus. It now occupies (after treatment) a cavity in the granular collenchyma. In 
the next stage, when the actines are about 0'06 mm. long, the characteristic form of the 
central part of the adult spicule has already originated. The young actines have now the 
form of triangular daggers with hollow faces (PL XXIX. figs. 8, 8a-6), and maybe readily 
represented by folding a triangle of paper into a three-sided pyramid, and pinching in 
the sides ; if four such pyramids be joined together at their bases, so that their edges 
are confluent, a model of the young spicule will be produced. The model will be 
exact except in one particular, it will not represent the webbing of the angles of the 
3 ^oung spicule produced by an extension outwards of the confluent ridges. This feature 
may readily be added to the model by gumming triangles of paper with a concave 
base across the angles, so that the concave curve continues the direction of the ridges. 
The young spicule still presents a deeply eroded surface, the pits frequently perforating 
it, so that it looks almost like a network of silica rather than a solid structure. With 
further growth the erosion disappears, the surface becomes smooth and even, the sharp 
edges of the actines thicken into rounded ridges, the triradiate depressions about the 
centre become circumscribed, and finally, towards the distal extremities, the actines or 
epactines, as we may now call them, assume a c^dindrical form, and, sometimes bifurcating, 
sometimes not, break out into the tubercles characteristic of the adult. The tubercles 
are at first slender, somewhat twig-like processes, and their intergrowth takes place in 
the following manner : — The contiguous epactines or their cladi are covered with tubercles 
on their apposed faces, and owing to their numbers, proximity, and direction of growth, 
they cannot enlarge unhindered — they are constantly encountering one another. A very 
general case is where one tubercle in its forward growth is hindered by another growing, 
at right angles to it, so that the- side of the latter is opposed to the growing end of the 
former ; this in consequence subdivides, or becomes bifid, and continuing its onward 
direction it encloses its opponent on each side. The increase in thickness of both the 
enclosing tubercle and its opponent, which it has taken prisoner, leads to a mutual 
adaptation of the closest kind, though, however close it may become, a thin film of tissue 
