Axial Canals of Sponge Spicules, (&c. By Prof. Duncan. 565 



is open at both ends, and has a shght enlargement near the extre- 

 mities. One part of the end of the spicule has been eroded off. 



It is evident, that in nine forms of sponge-spicula taken from 

 one sounding under conditions of great pressure and cold, remark- 

 able enlargements of the axial canals are observable. 



The symmetry and elegance of the outline of the enlargements 

 are without exception, and the usual shape is elongate cylindro- 

 conical, the base being convex in a single or double curve. The 

 position of the base varies ; often the cones are placed in more or 

 less apposition by their points, but the reverse is seen also. 

 Evidently the curves of the enlargements, as seen in outhne, do not 

 relate to the lines of the original deposit of the silica. 



In some instances the enlargement is filled, or partly filled, 

 with air, and in the others Canada balsam has flowed in ; and 

 whilst in some the parietes of the enlarged spaces are smooth and 

 glassy, in others they are minutely punctate, and present a worn 

 appearance. In some enlargements there is a greenish tinge and 

 distinct granular matter. 



The careful examination of a great many spicula proves that 

 specimens of every typical spicule, whether it be acuate, acerate, 

 basally-rectangulate, spinulate, dirhophalate, basally or entirely 

 spined, may have different kinds of enlargements of the axial canal. 

 In some instances the normal condition exists. 



Hence the enlargements are abnormal, and subsequent to the 

 development of the spicule. 



In investigating the possible causes of these enlargements, I 

 was led to examine the spicula with higher powers. Three other 

 structural changes, which have already been alluded to in the 

 descriptions, were then carefully noticed in them, and it appears 

 that they relate to the enlargement of the axial canals. 



First. The axial canal, instead of ending a short distance from 

 the end or ends of the spicula, may be open at one or both extre- 

 mities. This condition is usually accompanied by simple enlarge- 

 ment of the axial canal near the end, or shght general and equal 

 enlargement of the cahbre of the canal. But in many of the 

 spicula under consideration, there are constrictions at different 

 distances up the canal, and conoidal enlargements. 



Secondly. Minute cylindrical perforations occur, passing from 

 the outside of the spicule through the siliceous structure to reach 

 the axial canal. They are usually perpendicular to the path of the 

 canal, are very narrow, and are straight. Sometimes a circular pit 

 on the surface denotes one of these in the commencing stage, or 

 one may have only got through one-third or one-half of the way 

 to the axis. 



In some specimens the outer surface of the spicule has a dull 

 and worm-eaten appearance from the presence of numerous separate 



