XII 



PHYLUM MOLLUSCA 



703 



placed at regular intervals, and in this way the outer and 

 inner 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 simple lamina, each 

 formed of an outer and an inner lamella : the inner lamella of the 

 outer and the outer lamella of the inner laminae are united along 

 their dorsal edges, the line of junction representing the axis of the 





pap 



FIG. 586. Adult specimen of Nucula delphinodonta, represented as seen from the right 

 side. Reconstructed to show internal organs. Fully gro\vn specimens are 4 mm. long. 

 aa. anterior adductor muscle ; bg. byssal gland ; eg. cerebral ganglion ; /. foot ; ff. gill ; 

 h. heart ; int. intestine ; Ip. labial palp ; ces. oesophagus ; ot. statocyst ; pa. posterior adductoi 

 muscle; pap. palp-appendage; pg. pedal ganglion; sto, stomach; vg. visceral ganglion. 

 (After Drew.) 



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

 of the inner laminae are free dorsally. 



In Mylilus (Fig. 587, D) the gill is strengthened by the develop- 

 ment of delicate non-vascular bars or interlamellar junctions 

 between the two limbs of each filament. In Lucina these 

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

 bars of tissue, the intern' lamen tar junctions, replace the ciliary 

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

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



