80 PORIFERA. 



into the body of a Foraminiferan. But although the choanocytes 

 may have this ingestive capacity, their main function is probably to 

 cause currents through the canal system. 



The question of the ingestion of solid food by sponges has been much disputed. 

 There can be no doubt that solid bodies are introduced into the mesoderm cells, 

 for apart from the fact that foreign bodies such as sand-grains, etc. are found in 

 the mesoderm cells and spongin fibres of many sponges, Metschnikoff and 

 other observers have found carmine particles in carmine-fed sponges. It is not, 

 however, certain that the flat epith^elia co-operate with the choanocytes in this 

 introduction. Metschnikoff (loc. cit., p. 372) states that Halisarca, when 

 overfed with carmine, loses its canals and becomes a mass of amoeboid cells 

 containing swallowed matter and surrounded by a common envelope of ectoderm. 

 The same fact has been observed by Lieberkiihn* in Spongilla in winter. From 

 these observations, and others by Haeckel and Carter, it appears that under 

 certain nutritive conditions the cboanocytes may lose their flagella and collars 

 (according to Topsent and others these structures are retractile like the pseudo- 

 pods of an Amoeba) and become amoeboid, and the whole sponge may revert to 

 the condition of the larva of Aplysitia (F. E. Schulze, Z. f. w. Z. xxx. PI. 24, 

 Fig. 30), of a protoplasmic network with nuclei at the nodes surrounded by 

 a cortical layer of ectoderm. 



The collared cells are thus inconstant, and appear to be merely parenchyma 

 cells specially modified and capable, under certain nutritive conditions, of passing 

 back to their original form. When they vanish the canal system also goes, and 

 the sponge becomes solid so far as the latter is concerned. Inasmuch as the 

 parenchyma cells and the ectoderm cells are all connected by their processes 

 (except in the cases in which they break away and become amoeboid), it is clear 

 that the sponge in this condition, and in the case of Schulze's larva already 

 referred to, is but little more than a multinucleated Protozoon, differing from 

 the latter in the greater development of the vacuoles of the central portions, and 

 in the presence of a distinct cortical layer of nuclei. 



Skeleton. Skeletal structures are found in almost all sponges 

 (absent in certain Hexaceratina and Carnosa\ and are of considerable 

 importance in the classification. They may consist of calcareous 

 spicules, of silicious spicules, or of spongin. Spongin is a horny, 

 substance, resembling silk in chemical composition. It is usually 

 found in the form of fibres connecting together the silicious spicules 

 (many Monaxoriida), or forming the entire skeleton (Ceratosa). In 

 a few cases it is present as separate horny spicules (Danoinella). 

 In the Ceratosa, and probably in other sponges, it is secreted in 

 concentric layers by a number of mesodermal cells, called spongoblasts, 

 which are found coating the fibres (Fig. 70). In some cases the fibres 

 enclose foreign bodies, such as sand grains. In the Monaxonida the 

 amount of spongin present is, roughly speaking, inversely propor- 

 tional to the number of spicules, and there are all variations between 



* "Beitrage zur Entw. d. Spongillen." Mullens, Arch., 1856. 



