654 ZOOLOGY SECT. 



limbs of the filaments are respectively joined together, so as to 

 convert each longitudinal row of U-shaped filaments into a double 

 plate, fairly coherent unless the ciliary junctions are forcibly 

 .separated. In this way the single ctenidium of Nucula has given 

 place to two plate-like gills, each formed of an outer and an 

 inner lamella : the inner lamella of the outer and the outer 

 lamella of the inner gill are united along their dorsal edges, the. 

 line of junction representing the axis of the ctenidium : the 

 outer "lamella of the outer and the inner lamella of the inner gill 

 -are free dorsal ly. 



In Mytilus (Fig. 549, D) the gill is strengthened by the "develop- 

 ment of delicate non-vascular bars or inter-lamellar junctions 



Son, 



** 



TIG. 548. Nucula nucleus, the animal with' the left mantle-lobe removed ; a. ad. anterior 

 adductor; ar, anterior retractor ; ft. foot; gl. gill; r/on. gonad ; /. levator ; m. right lobe of 

 mantle ; mth. mouth ; p. ad. posterior adductor ; pip. palp ; p. r. posterior retractor. (After 

 Pelseneer.) 



between the two limbs of each filament. In Lucina these 

 junctions are large and provided with blood-vessels, and vascular 

 bars of tissue, the inter-filamentar junctions, replace the ciliary 

 junctions of the lower forms. Thus by a regular series of grada- 

 tions the ctenidium is replaced by the complex double gill we are 

 already familiar with in Anodonta. In all the higher forms the 

 outer lamella of the outer gill concresces with the mantle and the 

 inner lamella of the inner gill with the visceral mass, while, 

 posterior to the latter, the inner lamellae of the right and left 

 inner gills unite with one another. The blood-vessels, which are 

 confined to the filaments in the simpler types, occur only in the 

 inter-filamentar and inter-lamellar junctions in the more complex 

 forms of gill. In the Septibranchia the gills are degenerate, 

 being represented by a horizontal muscular partition or septum 



