Axial Oanah of Sfonge 8;picules, &G. By Prof. Duncan. 569 



whicli there is sucli a singular variety in tlie deposit, did not grow 

 where they were found ; many are moderately deep-water and 

 some shallow-water forms, and all have (in all probability) been 

 washed here and there, and have sunk down into the abyss. 



Firstly, it is to be noticed that many spicula have multitudes of 

 circular markings or depressions on them, which are concave 

 towards the centre of the spicule. In some, where there are a few 

 of these depressions, the circular outline is very distinct, and they 

 are never elevated, but are erosions in the outer layer of silica. In 

 several of the depressions minute elongate bodies are to be seen 

 (Fig. 16a), closely resembhng the non-ciliated zoospores oi Achlya 

 when immature or contracted after drying. 



In one specimen a distinct piece of a greenish cell projects 

 from the depression, which had become a tube reaching some 

 distance inwards. Several spicula have on them faintly green 

 bodies, circular in outline, very small in height, and with a faint, 

 darker central spot. These occupy the position and are of the 

 dimensions of circular depressions about to be. They project, of 

 course, higher than the spicule at a side view, and cover it when 

 seen from above (Fig. 16&). They singularly resemble the 

 zoospores of Aclilya penetrans* 



Within the enlarged axial canals, especially in two spinulates, 

 there are minute granules in a plasma. In one (Fig. 16c) four or 

 more granules are in the globular enlargement of the bulb, and a 

 minute granulation is seen in the canal. In the other, the granules 

 are smaller, but are aggregated into groups (Fig. 16d). 



In many axial canals there is a substance which refracts light 

 differently to the silica of the spicule and the surrounding balsam. 

 It is pale green in colour, and infinitely minutely granular. It 

 may contain refracting bodies like very small nuclei. 



Many axial canals have a worm-eaten appearance on their sides, 

 and the silica is rotten for some distance, producing irregular 

 enlargement (Figs. 4&, 4c, 5|, 12a), as if there had been special 

 points of erosion from within, and granules are seen in the hollows 

 in the sides of the canal. 



All these observations tend to enhance the hypothesis that the 

 penetrations from without are produced by a very lowly organized 

 plant ; that when penetration into the axial canal has taken place, 

 the organism fills it and proceeds with its erosion, enlarging the 

 calibre ; and that sea water, making its way in, with or without the 

 assistance of the Thallophyte, will gradually increase the diameter 

 and length of the canal. After a while thinning occurs and 

 rupture of the spicule, or it may be so drilled as to become dis- 

 continuous. Evidently there is no relation between the shapes of 

 the enlargements of the axial canals thus produced, and those of the 



* Loc. cit., pi, vii., fig. 61. 



