PHYSIOLOGICAL SERIES. 



HYDHOZOA. 



The supporting framework (corallum) of the following forms 

 is secreted by the outer layer of the body (ectoderm) upon its 

 external surface. It is accordingly outside the body, but since 

 the latter remains closely investing and surrounding the corallurn 

 the coral functions as an internal support to the animal. The 

 endoderm in many forms a cartilage-like support to the arms. 

 The "supporting lamella" and mesogloea likewise serve for 

 support. 



A. 13. Calcareous corallum of Millepora alcicornis (Order Hydro- 

 corallinse), and drawings illustrating its structure. The 

 corallum is secreted by the outer layer of cells (ectoderm) 

 of a network of tubes (hydrorhiza), and a continuous layer 

 of such cells on the outer surface of the coral. The hydro- 

 rhiza gives origin by budding to two sorts of zooids, which 

 can be retracted into tubular chambers (calicles). The 

 lower portion of the calicles are not occupied by the zooid, 

 and are cut off by successively formed plates (tabula). 

 Only that portion of the Millepora superficial to the last- 

 formed tabula is living. The interior of the hydrorhiza- 

 tubes are filled wdth a symbiotic alga (Zooxantliella 

 infeztans) in the form of small round yellow cells, having a 

 well marked cell-wall : they are quite free. 

 Moseley, Phil. Trans., vol. clxvi. 1877, p. 91. 



In the following diagrams the colours represent Black or 

 White = Corallum ; Brown = Ectoderm ; Blue = Mesoderm ; 

 Green = Endoderm. 



A. 14. Calcareous corallum of Stylaster roseus (Order Hydro- 

 coralline), and diagram showing its structure. The co- 

 rallum is secreted as in the former specimen. The arrange- 

 ment and shape of the calicles of the dactylozooids as a 

 circle of flattened wedge-shaped chambers surrounding 

 a central deeper and larger calicle in which the gastrozooid 

 is lodged, gives this coral a close superficial resemblance to 

 an Oculina (Class Anthozoa). A style-like process of the 



