442 JANE STEPHENS : ATLANTIC SPONGES 



in diameter, rising vertically from the base of the sponge to the surface. These are 

 placed about 0*1-0*15 mm. apart, and contain a good deal of spongin. The spicules 

 in these fibres are chiefly large styli, but towards the surface the columns end in 

 vertical brushes of slender dermal styli. These, as well as some of the large styli, 

 project beyond the surface of the sponge. Occasionally a large, isolated stylus, with 

 head based on the substratum, is seen between the fibres, but usually this space is 

 quite free from megascleres. The skeletal styli vary in size according to their 

 position in the fibres. The largest have their heads inserted towards the centre of 

 the fibre, while the smaller styli are placed externally to them and occur towards 

 the base of the fibre. 



The dermal skeleton consists of vertical brushes of slender styli. Similar styli 

 also lie horizontally in the dermal membrane between the skeletal fibres, and a few 

 are to be seen lying horizontally at the base of the sponge. 



Spicules. — (l) Styli of the main fibres, 0"ll-0"6 mm. in length by 0'01-0'03 mm. 

 in thickness. These vary in size according to their position, as already described. 

 The largest styli are slightly constricted immediately above the head ; the shaft is 

 curved and the spicule is quite smooth, or at most, is sometimes extremely minutely 

 spined on the head. The smallest styli are of a similar shape, but have a fairly 

 strongly spined head and a few small spines scattered along the shaft. They are not 

 very numerous. The longest and the shortest styli are not sharply marked off from 

 each other, but are connected by a series of styli intermediate in size and in degree of 

 spinulation. (2) Dermal styli, slender, with usually a slightly crooked shaft ; they 

 are very minutely spined on the head, and they vary in length between "2-0 '45 mm. 

 by 0*003-0*005 mm. (3) Isochelae palmatse of the usual Microciona type, 0'02 mm. 

 in length. (4) Toxa, evenly curved ; arms with slightly recurved tips ; very 

 uniform in size and shape ; 0*06-0*075 mm. in length by 0*002 mm. thick. 



The spicules of this species are somewhat similar in shape to those of Microciona 

 atrasanguinea Bowerbank, but their measurements are different. The large styli of 

 Microciona similis are twice as thick as those of M. atrasanguinea, while the toxa are 

 only half as long as those in the older species. The smallest styli of M. similis are 

 much larger and less spiny than the corresponding spicules of M. atrasanguinea. 

 In section the two species look very different, as in M. atrasanguinea the fibres are 

 much larger and thicker than in M. similis, and contain much more numerous styli. 



In spite of these differences, which lie chiefly in the proportions of the spicules, it 

 seems possible that a series of specimens might be obtained, showing intermediate 

 characters as regards the size of the spicules and thus bridging over the differences 

 between the typical M. atrasanguinea and the South African specimen above 

 described. On the other hand, specimens of M. atrasanguinea so far collected at 

 different parts of the coasts of the British Isles show very little variation in the size 

 of their spicules. For the present, at any rate, the South African sponge must be 

 regarded as a distinct species. 



