62 



SPONGES 



TABLE OF THE VARIOUS CLASSES OF CELLS. 



Dermal Layer 



I. Epithelial stratum 

 II. Porocytes . 

 III. Skeletogenous stratum 



1. Pinacocytes (epithelial 



cells}. 



2. Myocytes (contractile 



cells). 



3. Gland cells. 



4. Spongoblasts. 



5. Pore cells. 



6. Scleroblasts. 



7. Collencytes (stellate 



^Gastral Layer . | IV. Gastral epithelium 



Archaeocytes (pri- 

 mordial cells) 



V. Amoebocytes (wander- 

 ing cells) 



VI. Tokocytes (reproductive 

 cells) 



9. Cystencytes (bladder 

 cells}. 



10. Choanocytes (collar 



cells}. 



11. Phagocytes (ingestive 



cells).* 



12. Trophocytes (nutritive 



cells). 



13. Thesocytes (storage 



cells). 



14. Statocytes 'jemmule 



cells). 



15. Gonocytes 

 cells). 



(sexual 



Historical Review of Sponge Histology. The earlier observers by teasing 

 up sponges with needles saw amoeboid cells and sometimes ciliated cells. 

 The discovery of the resemblance of the latter to Choanoflagellata was 

 made by James-Clark (1867), who, like most of his contemporaries, con- 

 sidered sponges as Protozoan colonies. It was Leuckart (1854) who first 

 drew attention to the architecture of the sponge as a whole, and com- 

 pared it to a Coelenterate. Haeckel (1872) formalised this conception, 

 and termed the two layers composing the body wall dermal and gastral 

 respectively. His names are adopted here in the same sense. The 

 dermal layer, which he termed " exoderfn," and compared to the ectoderm 

 of Coelenterata, was regarded by him as a syncytium, made up of fused 

 cells, the protoplasm of which formed the clear ground substance of the 

 parenchyma, while the nuclei with a small quantity of protoplasm 

 formed the corpuscles. The spicules arose by crystallisation in the 

 ground substance, a condensation of which around the spicule formed 

 its sheath. The gastral layer (" entoderm ") consisted of the collar cells, 

 from which arose the ova and spermatozoa. 



Schulze in 1876 exposed the falsity of Haeckel's syncytium theory 

 by the disqovery of the flattened epithelium. Although this was a great 

 advance from the histological point of view, the conceptions of sponge 

 structure which Schulze founded upon it were less happy, and in many 

 respects further from the truth, than Haeckel's views. He considered the 

 flat epithelium to be partly ectoderm, partly endoderm, the collar cells 



1 It is possible that the phagocytes should be classified under the porocytes (see 

 above, ]>. 49, footnote). 



