628 SUMMARY OF CURRENT RESEARCHES RELATING TO 



the longitudinal axis, and divide the cavity in such a way that every two 

 segments enclose a lumen which opens above and to the exterior by 

 means of tho polyps and carries below a row of suckers. Each polyp 

 has a corresponding sucker, and we may, therefore, regard each polyp 

 plus a sucker, as one individual. Each sucker has a retort-shaped 

 cavity which communicates with the lumen of the colony. Typically 

 each polyp resembles an Actinian in minute structure. Cells and 

 fibres unite to form a continuous nervous sheath. The endodermal 

 cells are completely filled with parasitic plant-cells. There are no 

 radial or circular muscular fibres in the suckers, and they repeat the 

 general type of structure, with the exception that glands are developed 

 in them ; the septa have a double musculature ; when tlieir longitudinal 

 fibres contract the foot with the suckers is withdrawn from the ground, 

 and the transverse row of suckers which correspond to the partition 

 are set free. The transverse system of muscles next contracts, and the 

 whole colony becomes longer along its long axis; in this way the 

 movement of the colony is effected. 



Porifera. 



Sponge Spicules.* — Prof. W. J. Sollas discusses the possibility 

 of siliceous sponge spicules being transformed into calcareous spicules, 

 as has happened in fossil sponges. He finds the siliceous spicules 

 are composed, not of pure silica, like quartz, but of a colloid variety, 

 combined with organic material. In order to determine the refractive 

 index of these spicules, they should be placed in some fluid in which 

 they become invisible, that is in a fluid of the same refractive index. 

 The fluid in this case was chloroform, the index of which is 1 ■ 449 ; 

 thus these spicules are composed of a substance with very nearly the 

 same index as opal. By this method similar species of minerals can 

 be distinguished when isotropic; and even anisotropic substances can 

 be so distinguished, by using, in conjunction, Nicol's prisms. In the 

 case of fossil calcareous sponges, their preservation or not depends on 

 their being composed of calcite or arragonite or some combination of 

 either with organic substances. 



In order to ascertain the specific gravity of sponge spicules, the 

 author adopted a method which is described at p. 879, Vol. V. of this 

 Journal. From his experiments he considers them to be probably 

 calcite in combination with organic matter, their specific gravity being 

 2*62. The author disputes Hackel's view that regular triradiate 

 spicules have a crystalline form, derived from a regular 12-sided 

 pyramid ; he finds that the optic axes of a sagittal spicule and the 

 morphological axis of the unpaired ray are in the same plane, which 

 is a right angle to the plane of the spicule. 



From other experiments with crossed nicols he concludes that the 

 acerate is not homologous with the unpaired ray, but with one or both 

 of the paired rays, of a sagittal spicule. 



Calcareous spicules, after remaining in Canada balsam for some 



* Scientif. Proc. R. Dublin Soc, iv. (1885) pp. 374-92 (1 pi. aud 7 figs.). 



