CTENOPHORA 



137 



addition of a second adjacent quadrant. Accordingly the 

 Ctenophora are radiate animals ivith two rays (Fr. MiJLLER, 

 Clacjs). In this case it is impossible, though of no conse- 

 quence, to determine whether we ought to designate the 

 radii of the sagittal plane as perradii and those in the trans- 

 verse plane as interradii, or vice versa. Bj the unequal de- 

 velopment of organs Ijing in the plane of a secondary axis 

 the biradial structure may be converted into the bilaterally 

 symmetrical (for example, in the larval form known as Thoe 

 paradoxa by the development of a single tentacle). 



f / 



\ 



m ; 



'9 \\ ^ O 



Fig. 63. — Mertensia stage of Eucharis tnulticornis seen from the sensory pole 

 (after Chun), diagrammatic, aa, transverse axis ; bb, sagittal axis ; m, stomach ; 

 po, excretory pores ; p, polar plates of the apical sensory organ ; t, tentacular ap- 

 paratus ; mg, gastral vessels and r, meridional vessels in cross-section. 



The bilateral symmetry of two neighbouring quadrants of the Cteno- 

 phore suffers a certain derangement by the position of the two excretory 

 pores. For the infundibulum communicates with the exterior by means 

 of two openings situated in the vicinity of the apical pole (Fio. 63 po), 

 which lie in two diagonally opposite quadrants. This derangement is, 

 of course, to be explained as the result of the suppression of two pores, 

 for probably there was originally one pore present in each quadrant, 

 consequently four in all, a condition which, according to E. Heetwig 

 (No. 12, p. 318), is retained in Callianira bialata. There is no essential 

 change in the biradiate structural plan of the Ctenophora owing to this 

 asymmetrical development of the excretory pores, just as in the Bilateria, 

 for instance, an organ is frequently seen to develop asymmetrically with- 

 out the bilateral type being thereby destroyed (Claus). 



