Chap. I. STRUCTURAL FEATURES. 169 



of the senses are prolongations of the hiteral evolutions of the brain, comlnning 

 with involutions from the surface, while in the latter they are modifications of 

 the lateral ai^pendages of the annular elements of the body of these animals. 

 Whatever be the real functions of these controverted organs in the Radiates, one 

 thing is certain, — they are all modifications of the aml)ulacral tubes connected 

 with the ambulacral tentacles. This homology is readily ascertained by a careful 

 comparison of the so-called eye-specks of the Discophorne, especially those of Aurelia 

 (PI. IX. Fi(js. 3 and 4); and if the homology I have attempted to trace between 

 them and the eye-specks of the Echinoderms and the single speck of the Cte- 

 nophorte, in the centre of the circumscribed area, is correct, then all these organs, — 

 whether eye-specks or auditory bags, whether simple pigment cells upon the surface 

 of the tentacles, or vesicular cavities, including various concretions and apparently 

 independent of the system of tentacles, — are homologous modifications of one and 

 the same apparatus throughout the whole type of the Radiates, and constitute 

 organs with more or less specified functions, possibly analogous to the functions of 

 seeing and hearing in the other branches of the animal kingdom, but certainly 

 built upon a different plan, congruent with the idea of radiation, which pervades 

 them all. Gegenbaur states that he has looked in vain for the eye-speck in 

 Euramphaea. Is it possible that this genus should present such a departure from 

 the universal structure of its type ? It does not appear probable to me. 



Having thus flxr traced the special homologies of the sensitive organs connected 

 with the chymiferous system, I would suggest, that, if the comparisons I have 

 made are correct, it becomes prol>able that the circumscribed area of the abactinal 

 pole of the Ctenophoraj corresponds to the line encircling the dorsal surface of 

 the Star-fishes, or the narrow field included between the abactinal tennination of 

 the ambulacral zones in the Sea-urchins. Whether the fringes of the edge of the 

 circumscribed area of Beroe correspond in any way to the marginal tentacles of 

 the Discophorfe, as McCrady suggests, or not, I am not prepared to sa}^ If the 

 homology I assign to the area itself is correct, it would scarcely be possible to 

 homologize its marginal fringes with the marginal tentacles of the Discojjhorte, 

 since these are themselves homologous to the ambulacral suckers. 



Gegenbaur has correctly homologized the lateral auricles of the Mnemiidae, in 

 comparing them to the anterior and posterior lobes of that type, only he should 

 have added that there is, however, this structural diiference between them, that 

 while two spheromeres, with their ambulacral tulies and rows of locomotive flappers, 

 comliine on the anterior and on the posterior side of the spherosome, to form one 

 anterior and one posterior lobe, each lateral spheromere has its independent auricle, 

 so situated that while those of one side are the mates of those on the other side, 

 those of the same side stand also in antitropic relation to one another. Their 



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