STRUCTURE OF SUBERLTES DOMUNCULA. 245 



persistence of the organism in unfavourable environment, which can of course 

 only be verified by following their history. They might therefore be termed 

 regenerative capsules. Somewhat similar aggregations or contractions were 

 observed to occur in Renter a, but in these I was not able to make out any 

 definite structure. 



As a histological modification in response to a change in the environment, 

 whether it be connected or not with securing the persistence of the endangered 

 organism, the occurrence of these structures is perhaps worthy of record. 



Explanation of Plates. 

 Plate XVI. — Suberites. 



Fig. 1. External appearance, showing pores. 



2. Large section, showing spicules, canals, and ova, 



3. Ectodermic epithelium. 



i. System of canals, showing ciliated chambers. 



5-6. Muscle-cells. 



7-8. Connective tissue cells of mesoderm. 



9. Ova. 



10. Germinal vesicles with nucleoli. 



11. Sperm-morula?. 



Plate XVII. — Capsules of Spongelia. 



Fig. 1. Capsules on surface of sponge. 



2. Individual capsule on peak of sponge. 



3. Contents of capsule squeezed out. 



4. Longitudinal section. 



5. Border of capsule. 



6. Abundant round cells. 



7. Longitudinal section. 



8. Contained elements. 



9. Cross section. 



10. Longitudinal section with abundant round cells. 



11. Ectoderm outlines. 



VOL. XXXIII. PART. I. 2M 



