PORIFERA OR SPONGES. 



DIAGRAM V. 

 The Life-History of Sponges. 



The right half of the diagram (i-6) is after F. E. Schulze, and applies 

 to a calcareous sponge — Sycandra raphanus. 



(l) Represents the ovum; (2) is a side view of the stage with eight 

 cells ; (3) shows the blastosphere, with a few granular cells at the lower 

 pole ; (4) represents the free-swimming stage, the granular cells now 

 occupying one hemisphere ; (5) depicts the formation of the gastrula, 

 the ciliated cells becoming surrounded by the granular cells ; (6) shows 

 a gastrula which has just fixed itself mouth downwards : the ectoderm 

 or epiblast cells are amoeboid, the endoderm or hypoblast cells have lost 

 their cilia. 



The left half of the diagram (A-D) is after Heider, and refers to 

 Oscarella or Halisarca lobularis, one of the sponges without any hard 

 parts. 



(A) represents a section of a free-swimming ciliated blastula ; (B) 

 shows the invaginated gastrula ; at (C) the gastrula has fixed itself, and 

 the mouth or blastopore is being closed ; (D) is a diagrammatic section 

 of the young sponge, with radial chambers [r.c), with inhalent pores 

 such as that one marked by the entrant arrow, with an exhalent osculum 

 from which an arrow points outwards, and with mesoglcea developing 

 between ectoderm and endoderm. 



