210 CTENOPHOR.E. Part II. 



Ecliinoderms to ascertain their homologies, we must, as a matter of necessity, bring 

 them all into the same respective position, and contrast the arrangement of their 

 jiarts in their mutual correspondence. There is, however, no difficulty about their 

 identification, inasmuch as the mouth is made in every case the central point of 

 comparison. It has Ijeen already ascertained, that Polypi, though truly radiated 

 animals, have, in most of their types, if not in all, a rudimentary indication of a 

 longitudinal axis in the oblong form of their mouth, which is the first indication 

 of a bilateral symmetry in the animal kingdom, occurring even among the lowest 

 Radiata ; while in Echinoderms it rises higher and higher, and becomes so promi- 

 nent in the Spatangoids as to influence not only the general form, but even the 

 nundjer and arrangement of the internal parts, and the length and special develop- 

 ment of the external ajjpendages and of the andjulacral rows. 



The class of Acalepluv, which is intermediate between those of Polj-pi and 

 Echinoderms, holds in these respects also an intermediate position. In Ctenophora^, 

 we have a slightly compressed body and an oblong mouth. But the mouth may 

 open in a direction transverse to the elongation of the ))ody. The question there- 

 fore is, Does the mouth, with the plane which ])asses through it and the opposite 

 area, in tliis case indicate the length of the axis of the bod3% and divide it into a 

 right and a left half; and are therefore the tentacles lateral appendages, one on the 

 right side and the other on the left side, as we should consider them if we wei^e 

 to place the axis of the mouth in the same direction as the axis of the mouth 

 in Polypi ? or have we to consider the tentacles as arranged along the longitudinal 

 axis, one on the anterior, and the other on the posterior, extremity ? And in that 

 case, are the folds of the mouth rather the first indications of an upper and a lower 

 lip, — as Ave should consider tliem were we to compare the transverse position of 

 the mouth with the positit)n this opening assumes in the oblong symmetrical Echi- 

 noderms, in which the bilateral symmetry has been made prominent, — or have we 

 to view also the indication of bilateral symmetry among Polypi as a tendency to 

 such an arrangement of the two lips ? I think I can be positive in the case of 

 Polypi ; for in Actinia, as well as in Astrangia and many other Actinoid Polyps, 

 the oblong fold of the mouth has miequal angles; and it would be to suppose the 

 right and left angles of the mouth to be assymetrical, and the upper and the lower 

 lip to be identical, if we should not consider this split as running in the longitudinal 

 axis. And tliat it indicates really a longitudimd axis is shown hy the circumstance, 

 that fecal matters are discharged along the rounded angle of the oblong mouth, 

 opposite to which there is, in many Polypi, a tentacle of a peculiar form, and some- 

 times differing in color from the others. 



This being the case, are there reasons to view Pleurobrachia in a different 

 light ? Are the Ctenophora^ more nearly related to Echinoderms in their arrange- 



