No. 2.] EYES OF POLYCH^TOUS ANNELIDS. 21 1 



show indications of being formed by the fused ends of the 

 retinal cells. 



The invagination of the cuticle concerns at the most the 

 stalk of the lens ; the main mass is to be regarded as a part of 

 a thickening of the cuticle, or else as a fusion of cell ends abut- 

 ting against the cuticle. 



The whole eye is thus a spheroidal collection of elongated 

 epidermal cells ending in a clear mass that may be secreted, but 

 appears rather to be the modified ends of these cells. Such 

 cells agree essentially with that represented in Diagram i. 



A review of previous work upon such eyes enables us to 

 bring most of it into agreement with this conception, allowing 

 sufficient weight to the differences attributable to insufficient 

 material and imperfect methods. 



Some observations upon the formation of eyes in larvae and 

 in budded heads show that the eye is formed from the epidermis 

 with no invagination, that the epidermal cells become pigmented 

 and elongate, that the rods are the ends of these cells, that the 

 other refracting media appear next the cuticle and may be parts 

 of these same cells. 



The opinion that the nuclei of the larval eyes lie peripheral to 

 the pigment as in the adult is, however, not supported by previ- 

 ous descriptions. 



The origin of the eyes in the Alciopidae presents complica- 

 tions that may be regarded as secondary and unusual. 



Cases in which eyes are found buried in the brain and sepa- 

 rate from the surface are to be considered secondary, either the 

 results of degenerative reductions from the above type or else 

 retentions of larval characters with the addition of separation 

 from the cuticle. 



In conclusion we may restate the above facts and ideas with 

 the aid of the accompanying diagram illustrating the presump- 

 tive phyllogenetic history of the annelid eye. 



Primitively certain areas of epidermis became specially sensi- 

 tive to light, one or more cells. Such areas (i) contained cells 

 with clear receptive protoplasm near the cuticle, and probably 

 pigment also.^ 



^ This pigment may be regarded rather as one of the secondary concomitants of 

 special activity in translating vibrations than as a necessary antecedent to such special 

 activity. 



