648 



MK. R. T. GiJNTHER ON 



[June 18, 



Radial Canals and Radial Symmetry. — In all large collections 

 of Limnocnida, several individuals occur which exhibit numerical 

 variation in the radial canals. ( Cf. tables on pp. 648-650). Among 

 70 individuals of all ages brought home by Mr. Cunnington in 

 which the radial canals could be counted, 54 showed the typical 

 number of radial canals (4), 9 had 5 radial canals, 6 had 6, and 1 

 had 7 radial canals, so that as large a proportion as 24 per cent, 

 have 5 or more radial canals. In this connection it is interesting 

 to note that the numerical variation of the tentaculocysts of 

 Aurelia aurita was found by Browne (Biometrika, vol. i.) to 

 affect nearly as large a percentage (20-9-22'8 per cent.) of the 

 individuals showing the normal number. 



In many meduspe in which " supernumerary " radial canals occur, 

 the additional canals are clearly seen to be due to the bifurcation, 

 near the gastric cavity, of one or more of the normal number of 

 radial canals. In no specimen of Limnocnida, and sixteen were 

 examined, was this seen to be the case : all the radial canals pro- 

 ceeded independently from the gastric cavity to the circular canal ; 

 the confluence shown in text-fig. 173 being quite exceptional. 



As might be expected, the order of the appearance of tentacles 

 and the number of sense-organs tend to become more irregular 

 in the case of individuals with 5 or more radial canals. E. g., in 

 the case of three individuals of different ages, and 14, 7 and 

 4*5 mm, in diameter, the numbers of the sense-organs in each 

 fifth of the periphery were 



III a. 40 + 35 + 48 -f 40 + 41 = 204. 

 /. 19 + 21 + 21 + 22 -f 18 = 101. 



i. 10 + 6 + 6 + 6 + 



36. 



And so too, in the case of individuals with the normal number of 

 tentacles, tentacles of a particular order are often fully formed in 

 one quadrant befoi-e there is any sign of them in another. 



Such variations may be expected to be frequent in the case of 

 medusfe in which a numerically high grade of radial symmetry is 

 reached. 



Detailed Re2)ort on the individtml sj)ecimens of Limnocnida 

 collected hy Dr. Citnnington. 



Date: August 31, 1904. Locality: Niamkolo Bay. 



