esterly: eucalanus. 49 



somes, or rhabdome-iike bodies, and the manner of innervation 

 (the cells are not inverted) are also evidence along the same line. 

 But the facts at hand do not warrant an extended discussion of this 

 matter, nor at the present time more than a reference to the possibility 

 of such a relation. 



The more or less prevalent opinions as to the relation existing be- 

 tween the median eyes of the Crustacea and the eyes of lower forms, 

 such as flatworms, have been referred to previously. Anil it has been 

 shown, that, so far as the conditions in Eucalanus may be taken as 

 indicative, the reference of the median eye to those of flatworms, for 

 example, is not warranted. On the other hand, the organs of Claus 

 are believed to be in cverif essential point com]:)arable to the inverted 

 eyes of other invertebrates, more particularly to those of the annelids ; 

 yet they exhibit a close similarity in the structure of their cells to the 

 median eye. 



It cannot be maintained that a single character, such as the struc- 

 ture of visual cells, may in itself be reasonal)ly regarded as showing 

 racial affinity. Yet the facts that the cells of the organs of Claus in 

 Eucalanus are probably eyes and if so are of the inverted, subepithelial 

 type, seem to me to be evidence, along unsuspected lines, of the deriva- 

 tion of crustacean from annelidan stock, which as already mentioned, 

 has been rather generally, and on other grounds, looked upon as 

 probable (Lang, '88-94, p. 419). The character of the nerve-ter- 

 minations as such in the visual cells does not seem to me to be of 

 as fundamental importance as the other matters I have referred to. 

 I think it probable that even in the organs of Claus the neurofibrillae 

 are not in the form of a "Stiftchensaum," but the observations are 

 not based upon as clear conditions as in the cells of the median eye. 

 Whether a visual cell is inverted or not has always been regarded as 

 of M'ide significance, and when cells are met with that possess this 

 character, such as those of the organ of Claus, the meaning is worthy 

 of consideration. As regards the derivation of the visual organs of 

 one group of animals from those of another, Carriere ('85, p. 201) 

 is of opinion that all known facts are against it, and that, on the con- 

 trary, the visual organs are "convergent structures." Hesse (:02, 

 p. 643) says, with regard to Carriere's idea: "Wir konnen diesen 

 Aiisserungen nach dem Vorhergehenden unmoglich beistimmen" ; fur- 

 ther (Hesse, :02, p. 040) "Dass aber gegeniiber anderen Organen die 

 Sehorgane eine Ausnahmestellung einniihmen, derart, dass sie weinger 

 von einer Gruppe zur anderen vererbt wiirden (Carriere), muss auf 

 Grund der Thatsachen durchaus bestritten werden." But at the 



