xvi THE VOYAGE OF H.M.S. CHALLENGER. 



§ 20. Survey of the two ancestral branches of the Acalephse. 



I. First Ancestral branch of the Acalephse 



(without gastral tseniola, with ectodermal genitalia). 



Acaleplue ectocarpse (Intoeniolae). 



II. Second Ancestral branch of the Acalephse 



(with gastral tamiola and endodermal genitalia). 



Acalephce endocarpse (Tseniolata?). 



1. Hydropolyps (common ancestral group of all Aca- 



lephse, and first of all of the Intseniolae). 



2. Craspedota? (Hydroniedusa?, which have origin- 



ated from sessile Hydropolyps by adaptation 

 to a free-swimming mode of life). 



3. Ctenophorse (an early side branch of the Craspe- 



dots — Anthomedusae). 



4. Siphonophorre (swimming colonies of Craspedotas — 



Anthomedusas), with polymorphism of the per- 

 sonse. 



5. Scyphopolyps (ancestral group of the tamiolataa, 



derived from a branch of the Hydropolyps). 



6. Acraspeda? (Scyphomedusaj, originated from sessile 



Scyphopolyps by adaptation to a swim m ing 

 mode of life). 



7. Coralla (Anthozoa, the principal group of the 



sessile Tseniolata, probably sprung from several 

 branches of the Scyphopolyps). 



§ 21. Individuality of the Medusa. The tectological value, or the individual stage 

 of form of the fully developed and mature Medusa is in every case that of an 

 inarticulate transversely axonial (or radiate) persona (" persona inarticulata staur- 

 axonia "). Each Medusa has, therefore, a vertical principal axis (with oral and aboral 

 pole) and two or more horizontal transverse axes, perpendicular to it, with twice as 

 many radii. The number of these radii (usually four) corresponds to the number of 

 the radial sections or parameres which compose the body of the inarticulate persona and 

 which only touch by their axial edges in the vertical principal axis of the body. Those 

 Medusae which form colonies or cormi, undergo at the same time considerable organo- 

 logical differentiations in consequence of extensive polymorphism of their personse and 

 become separated from the true Medusse as a special class : the Siphonophora. 



§ 22. Fundamental form of the Medusa. In all Acraspedse and in the majority of 

 Craspedotae, the regular pyramid is the geometrical basis-form of the mature body, and 

 the principal axis of the Medusa body (which is vertical in the normal position of the 

 animal) is the axis of the pyramid ; its upper (aboral) pole corresponding to the point, 

 whilst its lower (oral) pole, with the oral opening, falls in the middle of the base of 

 the pyramid. The angles of the regular pyramid (at least four) are transected by 

 the primary transverse axes (with twice as many radii of the first order), whilst the 

 middle lines of the lateral surfaces of the pyramid correspond to the indifferent radii 

 of the second order. 



