TYPE MOLLUSOA. 337 



eye of Pecten, and below this again comes a pigment-layer 



{P9)- 



In a small number of forms, e.g. Area, peculiar compound 

 eyes are also found on the edge of the mantle. They form 

 slight rounded elevations and consist of a number of conical 

 retinal cells, each surrounded by a sheath of six cylindrical 

 pigment-cells. Each of these groups of retinal and pigment 

 cells is known as an ommatidium and is separated from the 

 adjoining ones by slender intermediate cells, so that on sur- 

 face view the composite character of the eye is very distinct. 



The nephridia (Fig. 149, ne) of the Pelecypoda are always 

 paired, and each consists of a tube bent upon itself lying im- 

 mediately beneath the pericardial cavity into which one of 

 the limbs opens (np^)-, while the other communicates with the 

 suprabranchial chamber {np'), and so with the exterior. In 

 the simplest forms the entire extent of both limbs is glandu- 

 lar, but in the majority the limb which opens to the exterior 

 loses its glandular character and surrounds to a certain ex- 

 tent the glandular or proximal limb. In addition to these 

 nephridia, frequently known as the organs of Bojanus, peri- 

 cardial glands are of common occurrence in all but the 

 simplest Pelecypods, and apparently assist the nephridia in 

 their excretory function. They are known also as Keber's 

 organs and consist either of outpouchings of the anterior 

 portion of the pericardial wall into the space between the 

 two walls of the mantle ( Unio, Venus) or of similar evagina- 

 tions of the walls of the auricles into the pericardial cavity 

 {Mytilus), both methods of formation usually being associated. 



The reproductive organs (Fig. 149, r) are paired, lying usu- 

 ally in the tissue forming the base of the foot, though extend- 

 ing in some cases into the lacunar spaces between tke walls 

 of the mantle {Mytilus). They are very richly branched and 

 usually contain in any one individual only ova or sperma- 

 tozoa, as the case may be, though a number of forms are 

 hermaphrodite— such, for example, as the members of the 

 genus Cyclas and some species of the genera Ostrea and 

 Pecten. The ducts which convey the reproductive elements 

 to the exterior open into the nephridia near their proximal 

 ends in Nucula and a few other primitive genera, but more 



