388 



sponges seems to be functionless, may in Receptaculites be transformed 

 into a canal for the transmission of fluids. But although the cavities of 

 all the tubes in Receptaculites communicate with each other through the 

 endorhinal canals, and perhaps through the stolons also, they may not 

 constitute a canal-system. The so-called tubes are extremely slender, 

 and may be solid in some species. 



On comparison it will be found that the general form of Receptaculites 

 and structure of its body-wall are almost precisely that of the seed-like 

 body that plays so important a part in the development of Spongilla. 

 This consists of a small ovate or spherical sac with an aperture on one 

 side leading into the cavity within. The enclosing wall consists of a coria- 

 ceous membrane on the outside of which there are arranged, perpendicu- 

 larly to the surface, numerous small birotulate spicula, exactly as the tubes 

 of Receptaculites are arranged on the endorhin. The outer extremities 

 of these spicula give off at right angles a number of small spines corre- 

 sponding to the stolons above figured. These spines coalesce, and (if I 

 understand the figures rightly) become connected together, so that they 

 form by their union a plate similar to that of Receptaculites, only that it 

 is hexagonal. The plates of all the spicula enlarge until all come into 

 contact, and thus an outer tesselated integument corresponding to the 

 ectorhin is formed. In this stage a section through the seed-like body 

 shews an inner integument (or endorhin) , and an outer plated integument 

 (or ectorhin), the two being separated and at the same time connected 

 by the pillar-like cylindrical shafts of the spicula representing the tubes of 

 Receptaculites. The space between the tubes is, according to some 

 authors, filled with a gelatinous silicious matter ; but Bowerbank says he 

 did not detect this substance in the specimens examined by him. This 

 little sac or cell is a Receptaculites in miniature,, and it is also one of the 

 embryonic stages of a sponge. When we consider that the full grown and 

 adult individuals of many of the long extinct tribes of animals never 

 attained in their structure a more advanced organization than that exhi- 

 bited by the embryos of orders living at the present day, it does not seem 

 surprising that we should find in the palaeozoic rocks a sponge which 

 although often of large size, never became more highly developed, than is 

 the recent genus Spongilla, when it has only advanced to the sac-like 

 stage above described. It is not intended to assert here positively that 

 Receptaculites is a sponge, or to determine the question of its zoological 

 rank one way or the other, but only to direct attention to such peculiari- 

 ties in its structure as appear to me worthy of being taken into account in 

 the investigation. 



