612 SUMMAKY OF CURRENT RESEARCHES RELATING TO 



spersed with small, chiefly fusiform cells which Haeckel seems to 

 have taken for nuclei, having remarked on the comparatively slight 

 readiness with which they become stained. Some of the cells possess 

 large, clear spaces, which are perhaps vacuoles ; the amoeboid cells 

 occurring amongst the others are true ova; spermatozoa have been, 

 with some probability, detected in the form of agglomerated masses. 

 Thus it is probable that both kinds of sexual products proceed from 

 the mesoderm. The laminse composing the spicula are irregular in 

 thickness at different parts, contrary to Haeckel's assertion ; they 

 possess a distinct sheath, in which, however, no nuclei or structure 

 were detected, (c) The ciliated epithelium constitutes the third layer, 

 and includes only the collar-cells of the chambers ; the collars of 

 these are chiefly narrow and tubular, and appear, by the frequently 

 compressed and contracted state in which they are found, to be 

 specially sensitive. 



Afftnity of the Leucones with the other Calcisponges. — The Ascones 

 appear to give rise to the Sycones, as Haeckel has said, by invagina- 

 tion of the gastral layer with its collar-cells at various points, forming 

 small radial tubes ; but a study of the relations of the quadriradiate 

 spicula of the radial tubes seems to show that it is the gastral (i. e. 

 internal) opening of the tube which is homologous with the osculum 

 of Ascon, But it seems probable, not that each radial tube, but that 

 the entire Sycon is the equivalent of the primitive Ascon ; and further, 

 that the Sycon is not, by virtue of its intercanals and other pecu- 

 liarities, so distinct a type as Haeckel infers, but may be connected 

 with the Leucones. Thus the intercanals of the Sycones are probably 

 simply the spaces left between the invaginated radial tubes, and their 

 variations in size depend on the extent to which those tubes are 

 laterally united with one another ; thus it is probable that the inter- 

 canals represent the ^scow-pores or " pit-canals " of Haeckel. The 

 Leucon canal system appears to arise from that of the Sycones by the 

 formation of lateral caeca by the radial tubes, which become bound 

 together by a strong connective-tissue development, and the entoderm 

 or collar-cell layer becomes limited to small portions of the original 

 invagination. Thus (1) the radial tubes are imperfectly homologous 

 with ciliated chambers. (2) Sycon is derived from Ascon, and may 

 be modified to form Leucon. (3) A simple Leucon is the homologue 

 of a simple Sycon, and therefore with an Ascon ; each forming one 

 person, whose morphological centre is that space in each which is 

 lined by the entoderm cells. 



Point (1) is proved by the similar origin of the radial tubes and 

 ciliated chambers of Leucon, and supported by the fact that in some 

 cases even the former send out lateral caeca. 



The canal system of other — i. e. non-calcareous — sponges may be 

 derived from the Xewcow-type by the development of its ectodermal 

 invaginations into a system of fine canals, each ending in a ciliated 

 chamber. In fine, four main types of canal system must be assumed 

 to exist in the sponges. 



1. Ascon-tyl^c, the primary form ; the part which bears the collar- 

 cells opens directly to the exterior. 



