SPONGES 85 



and so forth, from the flagellated cells of the larva, which in their 

 turn are the earliest cells to be differentiated, and in the simplest 

 types compose the whole blastula with the exception of the archaeo- 

 cytes, the primitive germinal cells. The importance of these facts 

 from the point of view of phylogeny cannot be too strongly 

 emphasised. 



III. THE PHYSIOLOGY AND BIOLOGY OF SPONGES. 



The most important organ of the sponge, from the point of 

 view of metabolism and nutrition, is the canal system. During life 

 and activity the flagella of the collar cells keep up a constant flow 

 of water through the sponge. The current enters at the pores or 

 ostia, streams through the canal system into the gastral cavity, and 

 passes out by the osculum. From the incoming current the sponge 

 obtains its nourishment and a supply of oxyen for respiration ; by 

 the outgoing current the waste products of metabolism are removed 

 from the body. 



Although, however, the problem might seem a simple one, there is no 

 question which has been so much discussed as the nutrition of sponges. 

 The confusion that prevails is very largely due to imperfect knowledge of 

 the structure of the sponge body. Since sponges are a group in which 

 the cells are largely lacking in co-ordination and show a corresponding 

 independence of action, it is evident that here physiology must to a great 

 extent wait upon histology, and that a clear understanding of the latter is 

 necessary before it is possible to form coherent ideas about the former. 

 Hitherto advances in the physiology of sponge nutrition have been greatly 

 hampered by an indiscriminate use of the word " mesoderm." Since 

 under this term are commonly included cells so different in their nature 

 as porocytes, skeletogenous cells, and amoebocytes, it is clear that not 

 much is gained by ascribing this, that, or the other function to " mesoderm 

 cells." 



With regard to the ingestion of food two opposite opinions have 

 prevailed, one set of investigators attributing an ingestive function to 

 the collar cells, another set regarding the " mesoderm cells " as the true 

 phagocytes. Those who hold the former view explain the presence of 

 ingested particles in mesoderm cells as having been passed on to them 

 by the collar cells. The true explanation seems to lie, as Metschnikoff 

 (1892) has pointed out, between these two opinions. The "mesoderm" 

 shows a great difference as regards its degree of evolution in different 

 types. While in some, e.g. Ascons, the parenchyma is scarcely developed, 

 in others it reaches a high grade of complication. In accordance with 

 these differences the part played by the parenchyma in capturing food 

 may, in some cases, be very slight, in others very great. 



There can be no doubt whatever, from the numerous experiments 

 that have been performed by various investigators, from Carter and 

 Lieberkiihn in the fifties up to Vosmaer and Pekelharing at the present 



