CTENOPHOR^. 9 



apparently uberraiit types, constantly met with among animals, has 

 been the main cause of our difficulty in referring them to their proper 

 plan. It is not always an easy matter to reduce an equation to its 

 simplest form, and fnid out what it is ; it may be concealed by coef- 

 ficients which will disappear only after repeated operations, and then 

 only enable us to determine of what degree the equation is. These 

 coefficients in an equation may be compared to the modifications of 

 those parts which appear to affect the mode of execution in animals ; 

 and it may not always be an easy matter nor a possible one, in the 

 present state of our knowledge, to solve these organic equations. The 

 history of Science is full of examples of this kind ; and we may have 

 to discover new methods in Natural History, as well as in Mathematics, 

 before we can proceed with our eliminations, or arrive at a solution. 

 Thus the plan of radiation may be so carried out, by a modification 

 of some of the parts, as to appear at first sight to be bilateral ; but 

 analyze these modifications carefully, and beneath them all can be 

 traced the plan of radiation, hidden only by external features of bilater- 

 ality. Such is eminently the case in the larvjB of Echinoderms, and to 

 a less degree in the imitations of Echinoderm larvae, the Ctenophorae. 

 Bilaterality seems at first sight to be the plan upon which these animals 

 are built; but an elimination of the deceptive coefficients will show the 

 plan of radiation underlying this apparent bilaterality. 



The figures here given of very young Ctenophorae show no indica- 

 tion of this bilaterality, at least no more than can be traced in any 

 four-rayed jelly-fish. The tubes are as yet all of equal size, no promi- 

 nence is given to one side over the other, and the only hint of bilater- 

 ality is the early distinction of the longitudinal and of the transverse 

 axis by the position of the tentacles. No lateral appendages develop- 

 ing into immense lobes, as in the adults, can as yet be detected. The 

 characteristic feature of the eggs of the Ctenophorae is the great diam- 

 eter of tlie envelope compared to the yolk, which is hardly more than 

 one third the diameter of the egg. The whole yolk is transformed by 

 segmentation into the embryo ; this at an early period assumes a very 

 slightly pear-shaped form, and is moved by means of a few pairs of 

 large locomotive combs, equalling in length the diameter of the em- 

 bryos. This is the first indication we have that the embryo is a Cteno- 

 phore ; and the early stages are marked by the constant and violent 

 flapping of the combs, arranged in four bunches near the abactinal 

 pole, immediately at the base of the large eye, also disproportionately 

 large in the young, containing but few granules, and seeming almost 

 like a glass ball fastened to the top of this active embryo. During this 

 stage the young Ctenophore is moving about somewhat slowly within 

 the envelope of the egg. With increasing age the locomotive flappers 

 descend somewhat along the spheromeres, and we find at the opposite 



NO. II. 2 



