160 CTENOPHORiE. Part II. 



in the Actinoids, in which the dig-estive cavity is compressed in the same manner 

 as in the Ctenophonu, one end of the actinostome diflers in form and structure 

 and functions from the other end, -which amounts to a difference between the 

 two ends of the compressed digestive cavity, analogous to the difference existing 

 l)etween the odd aml)nlacr;il and the odd interamljuhicral zone at the two ends 

 of the antero-posterior diameter of tlie Echinoderms. I do not, therefore, hesitate 

 in considering that transverse diameter of the Ctenophoras wdiich coincides with 

 tlie h)nger diameter of the actinostome and of the eii'cumscrilied area as the 

 longitndinal diameter of these animals, and that which traverses the body in the 

 direction of the intermediate chymiferons tubes as the lateral diameter, and these 

 tubes therefore as an interamlnilacral structure homologous to the interaniljidacral 

 vesicles or tubes of the aquiferous S3'stem of the Echinoderms, and not homologous 

 to the radiating chymiferous tubes nor to the amljulacral tubes proper. As soon 

 as these comparisons are admitted as correct, it must lie als(j acknowledged, further, 

 that one of the leading peculiarities of the Radiates consists in the position of 

 the month, which, instead of appearing at the anterior end of the longitudinal or 

 antero-posterior diameter, is placed at the actinal end of the vertical diameter, or, 

 in other words, in the centre of radiation of the whole structure. 



The special structure of the Ctenophora3 readily accounts for their peculiar 

 symmetr}'. Built up of eight homologous segments, their spheroidal Ijody would 

 approach much nearer to a sphere, the primary form of all Eadiates, were these 

 segments or spheromeres nt)t unecjual among themselves in certain directions, and 

 again perfectly identical in every respect in other directions. Had the similarity 

 of the structure of the Acaleplis and Echinoderms been sooner traced in its details, 

 — had, esjiecially, the repetition of homologous segments around the vertical axis 

 of the Acalephs, and the homology of these segments and the amlnilacral zones 

 of the Echinoderms, been perceived, — it would have been easy to recognize the 

 foundation of their resemblance as well as that of their difference. The typical 

 architecture of the Echinoderms depends \ipon the presence of five homologous 

 zones, occasionally reduced to four, and sometimes increased to a larger number ; 

 while that of the Ctenophorai is based upon eight homologous segments. These 

 parts are distinguished Ity special homologies in their respective classes, Ijut present 

 an unmistakable general homology when compared to one another. When tracing 

 these general homologies, it must, however, be rememljered, that the distinction of 

 anibulacral and interandjulacral zones, introduced in the characteristics of the Echi- 

 noderms, should be discarded to the extent to which the}- merely exj^ress a speciali- 

 zation of parts peculiar to that class, since, in the Holothurians, the interambulacral 

 zones are not more distinct than in the Ctenophora^. 



Recalling now to om' mind the statement made before, that the body of the 



