THE HYDROMEDUSAE 



43 



Fio. 48. 



48. Diagram of the structure of Velella, showing the central and peripheral thirds of 

 a half-section of the colony, the middle third being omitted. The ectoderm is indicated by 

 close hatching, the endoderm by light hatching, the mesogloea by thick black lines, the horny 

 skeleton of the pneumatophore and sail by dotting. BL, blastostyle ; C, centradenia ; D, dacty- 

 lozooid ; EC, edge of colony, prolonged beyond the pneumatophore ; G, cavity of the large 

 central gastrozooid ; M, medusoids attached to blastostyles ; PN, primary central chamber, and 

 PN', a concentric chamber of the pneumatophore, the former showing the opening into the 

 second chamber, the latter showing an opening to the exterior, and a " trachea " ; S, sail. 



48a. Porpita from the aboral aspect, showing the pneumatophore, and expanded dactylo- 

 zooids. (After Agassiz.) 



