Il6 H. H. NEWMAN. 



posterior outgrowth of the left anterior ccelom. In Patiria the 

 posterior coelom typically arises on the left side of the archenteron 

 at about the level of the middle of the prospective stomach. It 

 is first seen in the form of a thick-walled evagination of the wall of 

 the archenteron (Fig. 7). This pouch does not expand into a 

 thin-walled vesicle, but merely constricts off a small solid ball of 

 cells that later expands and forms a lumen. The vesicle is always 

 a conspicuous object even under low powers of the microscope, 

 and it affords a ready means of determining the asymmetry of the 

 larva before there are any other positive indications of asymmetry 

 Variations in the Position of the Posterior Ccelom. The typical 

 point of origin of this organ is noticeably to the left of the median 

 line of the archenteron. In many cases, however, it occurs 

 exactly in the median line, under which circumstances it is not 

 infrequently completely double or at least more or less constricted 

 down the mid-dorsal line. Sometimes the double vesicle fuses 

 into one and moves sometimes to the right, but usually to the left, 

 and gives rise to right-handed or left-handed asymmetry. A 

 statistical study of large numbers of larvae revealed the pro- 

 portions in which right-handed and left-handed individuals occur. 

 Out of 916 specimens examined, 811 had the posterior ccelom on 

 the left, 98 had it on the right, and 9 had it in a median position 

 or so nearly so that no asymmetry could be made out. This 

 means that over 88 per cent, exhibit left-handed asymmetry, 

 over 10 per cent, right-handed asymmetry, and a little over i per 

 cent, show no asymmetry. This is in contrast with conditions 

 described by Gemmill (1915) for Porania pulvillus, where "a 

 rudimentary enteroccelic outgrowth arises dorsally by prolifer- 

 ation of the stomach endoderm, usually on both sides of the 

 median line, but sometimes in the median line, or on one side 

 only. . . . When only one body is present it occurs rather of tener 

 on the right than on the left side, a contrast to the condition in 

 Asterias rubens, where the corresponding body (a still smaller one) 

 appears most commonly on the left side." It seems evident from 

 these observations that the presence among the asteroids of the 

 posterior coelomic vesicle and its asymmetry exhibits a highly 

 variable incidence. The body is one that seems to be relatively 

 vestigial in the group as a whole, but appears in a Cribrella, 



