THE SCYPHOMEDUSAE 



eight brachial canals, the grooves of Semostomae (Fig. 1 3) ; these 

 canals unite into a manubrial cavity, which may either open 

 directly into the gastric cavity (Rhizostoma), or, owing to the 

 encroachment of the subgenital pits and the diagonal fusion 

 across its opening of the four strong pillars which support the 

 bases of the arms, may communicate with the gastric cavity only 

 by four perradial ^i/kr canals (Cannorhiza, Figs. 12, 13). 



The gastric cavity of the Discomedusae is generally broad and 



V\'\ 1.;. 



Diagram of a lonsitiulinal section of Cannorliiza. 



Lettering for both figures : ah, pcrradial arm pillar in Fig. 12, adradial arm in Fig. 13 ; ah, 

 mass of tissue formed by the concrescence of the arm pillars ; an, suctorial mouths along the 

 " oral " faces of the arms ; up, perradial arm pillar ; ch, brachial canal, formed by concrescence 

 of its lips over the brachial groove of Semostomae ; «•, circular canal ; «/, arm-pillar canal ; 

 ci, interradial canal ; cp, perradial canal ; ga, chamber formed by the union of the brachial 

 canals — the site of the mouth of Semostomae is immediately under the end of the reference line ; 

 gc, gastric cavity, cut off from ga by the encroachment of the four subgenital 'pits ami their 

 union into the subgenital porticus ; gg, gh, gastro-genital membrane, composed above of endo- 

 derm lining the gastric cavity and forming the generative organs, below of ectoderm lining the 

 subgenital iwrtieus, with niesogloea between the two ; ir, the subgenital porticus = the centrally 

 confluent subgenital pits, lined by ectodenn ; oi, interradial, and op, perradial, otocysts ; 

 s, endodemial generative organs on floor of gastric cavity ; um, margin of umbrella. (From 

 Ilaeckel.) 



shallow, and exhibits four interradial pouches, separated by the 

 four perradial arm pillars, strong ridges of thick mesogloea which 

 are continued into and support the arms. From these four 



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