217 



We have thus seen good reasons for holding that the nephrostome 

 is egestive in function and not excretory, and upon this hypothesis can 

 be explained the separate ontogeny of the nephrostome from the raeso- 

 blast, and its intimate connexion with the gonocytic function, whereas 

 the excretory function (see below) essentially belongs to the ectoderm 

 and there is no reason why it should be intimately connected with the 

 sexual function. The close union of sexual and urinary functions is 

 thus secondary. 



Before leaving the monocytic alimentary processes we can note 

 that it is from the immigrant ingestive cells that other monocytic or- 

 gans are evolved. 



The primary skeleton of Metazoa (this must be the only skeleton 

 present in such forms as Halisarca) 49 consists solely of indigestible 

 solid residua on their way to the exterior. In a slightly more differen- 

 tiated state the sclerocytes secrete their own skeletal structures (spi- 

 cules) but still continue their journey to the exterior. The spicules of 

 sponges are said to arise in the »mesodermal sclerocytes « and to travel 

 slowly toward the exterior ^o. 



If the travelling outwards be indefinitely delayed then a perma- 

 nent endoskeleton may arise, or again, the deposit may form an exo- 

 skeleton in the outer tissues of the body ^'. 



A similar delay in the extension of the gonocytes may be carried 

 on till late in life, and cause the phenomena of maturity, and again it 

 may take on a cyclic activity giving rise to a rhythmic repetition at 

 stated intervals, of the gonocytic discharge, just as a rhythmic discharge 

 of the result of sclerocytic activity gives rise to » ecdyses«. 



The connection of the monocytic egestive processes with pig- 

 mentary deposits has been pointed out by several observers •''2. In this 

 case also one can conceive that the pigments first entered with the 

 food, and were simple egestive residua, which travelled, like other 

 monocytic faeces to the exterior, the influence of light upon the pig- 

 ments, causing their direction to be towards the most intense illumi- 

 nation (Durham). As a matter of fact, the course of the egestive 



49 »Foreign bodies also contribute to the formation of the skeleton of some 

 siliceous sponges, and occasionally form the entire skeleton, no other hard parts 

 being present.« W. J. So lias, Encyc. Brit. Sponges. 



50 »During its growth the spicule slowy passes from the interior to the exterior 



of the sponge, and is finally cast out as an lefi'ete product.« W. Sollas, 



Encycl. Brit. Sponges p. 47. 



51 In the Ceratosa the skeleton is a polycytic secretion of spongin, which phy- 

 logenetically, has replaced the monocytic skeleton. A gradation of types shew this. 



52 H. Eisig,loc. cit. p. 18. — H. Durham, loc. cit. p. 21. He gives a biblio- 

 graphy of works upon this subject. — S. Harmer, loc. cit. p. 2G. 



VÓ 



