CHAPTER IV 



PHYLUM PORIFERA 



THE group of the sponges or Porifera occupies an almost isolated 

 position in the animal kingdom. Sponges agree, it is 

 true, with Coelenterata in exhibiting cellular structure 

 and having their protoplasm arranged in tissues ; and 

 further in the fact that all the internal cavities of the body are in 

 communication with one another, so that both Coelenterata and 

 Porifera might be described as systems of branched tubes. A 

 closer inspection however reveals the fact that the tissues of the 

 Porifera are very different from those of Coelenterata and originate 

 in a different way from the larva, so that the opinion is gaining 

 ground that whereas most, if not all, of the higher groups of 

 animals have descended from ancestors which had we seen them 

 we should have classed as Coelenterata, Porifera on the other hand 

 have been independently derived from Protozoa. 



In Coelenterata the colonies can be analysed into persons 

 (medusoids or hydroids) and stolons, and many of the Porifera show 

 a like aggregation of persons. But in many forms it is impossible 

 to suggest how many individuals are contained in the branch 

 system of a single aggregate since all distinctness of individuals is 

 lost. Further analysis shows that the apparent persons or units, 

 even when most clearly demarcated, are of very varying morpho- 

 logical value. 



The salient peculiarities of sponges will be best appreciated by 

 a short description of one of the simplest types known, 



Leucosolema. i, -, r 7 i . 



a sponge called Leucosolema, which is common on 

 most clean rocky shores. 



In this animal we can recognise a foundation consisting of a 

 network of horizontal stolons, adherent to some foreign object, 

 from which a number of upright tubes spring. Each upright tube 

 ends in a large opening, termed the osculum (1, Fig. 36), which 

 can be closed if the animal be irritated and which in Leucosolenia is 



