440 GENERAL HISTOKY OF THE INEUSOBIA. 



Leydig' next proceeds to show that the single eye-specks, appearing only 

 as an accumulation of pigment- granules, are precisely homologous structures 

 with the reputed eyes of Cyclo;ps and Dajplinia among the Entomostraca, and 

 of Argulus, Artemia, and Branchipus among the Phyllopoda. In neither the 

 one nor the other does a lens, cornea, or capsule exist, although in a few (for 

 instance, in Notomiyiata Myrmeleo) a gUstening white substance is intermixed. 

 The single eye-spot of Brachionus, with its coalesced central segments, has 

 its counterpart in the eye of the larva of Cyclojps, and an e\4dent analogy 

 with that of Cychjysina, as also with that of Caligiis, in both which a refract- 

 ing lens makes its appearance, it is likewise similar in general conformation. 

 Wh.o then, asks Leydig, can advance any direct arguments against the 

 hypothesis that, by the medium of the pigment-granules in immediate con- 

 tiguity with the nei-ve-cells of the brain, without a refracting body, a percep- 

 tion of light is possible ? That the Rotatoria, on the contrary, may possess, 

 equally with the Crustacea, a refracting medium, is illustrated by the example 

 of Euclilanis unisetata. With reference to those species having two eyes, Leydig 

 has convinced himself of the presence of a lens in both in PterocUna, Ste- 

 phanops, Metopidia, Rotifer citrinus, and in R. macrurus ; and he thinks he has 

 seen one in the eyes of the young of Tuhkolaria, Melicerta, and Stephanoceros, 

 although the soft state of the parts and their indistinct outline render the 

 observation less certain. In the last-cited animals, when any trace of the 

 eye-pigment remains, none whatever of the crystalline lens is visible. Of 

 the other binocular Rotatoria, not mentioned, Leydig' s opinion is, that ana- 

 logy intimates the existence of a refracting medium, and their nature as true 

 eyes. The presence of a special horny skin or a particular capsule sur- 

 rounding the pigment is doubtful ; for the cuticle probably performs the office 

 of a cornea. 



Of the many-eyed Rotatoria, Leydig has particularly examined Eosphora 

 and Theorus. He finds Ehrenberg in error respecting Eosphom, which, in fact, 

 possesses a single eye- speck above the brain ; and what that natui'alist took 

 to be two clear eye-points on the frontal margin are merely intensely orange- 

 or yeUow- coloured spaces, which are at once seen to be viithout any affinity 

 with the other eye-specks. The eyes of Theorus are nothing more than oil- 

 drops within the stomach-glands. Ehrenberg, moreover, describes colourless 

 eyes, the visual nature of which may well be doubted. Although he has no 

 direct observation, Leydig believes that in Squamella the pigment is composed 

 of numerous portions disposed around a crystalline lens, and that the animal 

 may consequently be called many-eyed. 



The conclusions arrived at by Leydig are, " that the single-eyed species of 

 Rotatoria have, many of them, a refracting body in their eye-specks, which 

 are therefore true simple eyes, but that in most cases a lens is wanting, and 

 the specks are merely rudimentary eyes ; whilst in those with two eye spots, 

 each of them is, by the presence of a lens, an actual simple eye." 



Ehrenberg stated that eye-specks were entirely absent in several genera : 

 such were the doubtful Rotatoria Ptygura and Ichihydium, also Chcetonotus, 

 Cyphonautes, Tuhicolaria, Eiteroplea, Hydatina, Pleurotrocha, LepadelUy 

 Hydrias, Typhlina, and Noteus. With reference to TuhicoUria, Leydig 

 shows that in the young state this genus has two eye-specks ; of the other 

 exceptional forms, several have been insufficiently examined to found any cer- 

 tain statements upon. 



The curious fact of the disappearance of the eye- spots in several Rotatoria 

 has been already referred to. Examples occur in the genera Melicerta, Laci- 

 nularia, Floscidaria, Tubicolaria, and MegcdotrocJia. 



c. The Psychiccd Endowments of Rotatoria ai*e probably of the nature of in- 



