228 



THE VOYAGE OF H.M.S. CHALLENGER. 



layer (ectosome) of the Cliona, and is also penetrated through and through by the .sponge 

 (choanosome). The tubular canals penetrating the Coral in every direction ramify and 

 anastomose and appear on the surface at fairly regular intervals in the form of circular, 

 vertical pits.' Each of these pits is closed at the top by a thick, cushion -like pad of 

 sponge tissue. These cushions are merely portions of the general crust which the Cliona 

 forms on the surface of the corallum, Ijut whereas the crust is in most places only 

 about 0"2 to 0'25 mm. thick, in the cushion -like areas it attains a thickness of about 



0"4 mm. on the one side of the corallum, and 0'8 

 mm. on the other {vide annexed woodcut). On the 

 one surface, again (PL XXV. fig. 5), the pits are 

 very much more abundant than on the other (PI. 

 XXV. fig. 5a), and average about 2 mm. in diameter, 

 while the distance between two adjacent pits is 

 also about 2 mm. On this surface the cushions are 

 also much thicker than on the other i^vide supra), 

 and they present no appearance of perforation to the 

 naked eye, though there is commonly a slight de- 

 pression in the centre (PI. XXV. fig. 5c). On the 

 opposite surface the pits are fewer and less regularly 

 arranged, and the cushion-like pads of tissue have 

 each a small, but distinct, oscular perforation through 

 the centre (PI. XXV. fig. bh). Colour of the sponge in 

 spirit light brown. Oscula minute (PI. XXV. fig. 56, o 

 and woodcut Fig. 11, o), one in the centre of each 

 cushion-like area on one surface only of the sponge. Pores, narrow, slit-like perforations, 

 between vertical brushes of spicules, many in each cushion-like area on the opposite side 

 of the sponge (woodcut. Fig. 11, ^a). 



Skeleton. — The ectosome contains numerous, closely packed, tylostylote spicules, many 

 (? most) of which are arranged more or less vertically to the surface, beyond which their 

 apices project for a short distance. In the cushion-like areas on both surfaces the spicules 

 are especially abundant and more regularly placed than in other parts of the ectosome ; 

 around each osculum they form a slightly projecting fringe {vide woodcut, Fig. 11). 

 The skeleton in the main body of the sponge, i.e., in the canals in the corallum, is very 

 loose and scanty, consisting only of a few scattered sjjicules. 



Spicules. — Megasclera ; of one form only, viz., rather slender tylostyli (PI. XXIX. 

 fig. 8), with very well marked heads ; size about 0'32 by 0"0065 mm. 



This is an extremely interesting and well characterised species ; the most important 

 points about it are (1) that it completely envelops as well as bores into the organism 



1 Originally the calicular cavities of the Coral, of which the sponge has taken advantage. 



Fig. 11.— Cliona dismnilis. Vertical section of ,i 

 decalcified specimen ; 0,0, oscula; pa, pore-areas, 

 c, ectosome ; ic, inhalent canals ; ec, exhalent 

 canals, x 4. 



