TYPE MOLLUSCA. 331 



of the pinnae, or gill-filaments as they may be called, by hollow 

 processes, the " interlamellar junctions " (Fig. 148, G, il). 

 A still greater departure from the primitive condition 

 is found, however, in the greater number of existing Pelecy- 

 pods, consisting of a fusion of all the filaments of each lamella 

 into a plate (Fig. 148, D), small openings {p) only being left here 

 and there between adjacent filaments ; furthermore the inter- 

 lamellar junctions become very well developed, so that the 

 two lamellae of each gill become firmly united together to form 

 a plate, containing in the interior a cavity, the interlamellar 

 space. 



In addition to these various modifications which lead to 

 the formation of a true lamellate gill, the edge of the external 

 lamella of the outer plate fuses with the inner surface of the 

 mantle, and the internal lamella of the inner plate fuses sim- 

 ilarly with the side of the foot (Fig. 148, B), and the mantle- 

 cavity thus becomes divided into two chambers. Into the ven- 

 tral chamber the inhalent siphon opens, and the water which 

 enters by it passes through the openings left between the 

 filaments and so reaches the interlamellar spaces which com- 

 municate above with the dorsal or suprabranchial chamber 

 {sbr), whence it passes to the exterior throiigh the exhalent 

 siphon. In the region of the foot the suprabranchial cham- 

 ber is of course divided into two portions, one of which lies 

 on each side of the base of the foot, and each of these is again 

 divided longitudinally into an inner and an outer portion by 

 the line attachment of the gills to what may be considered 

 the roof of the mantle-cavity. Behind the foot the inner cavities 

 of the two sides unite and in some forms open ventrally into 

 the mantle-cavity proper ; in others, however, the inner lamellae 

 of the inner gill-plates fuse with one another along the middle 

 line so that a distinct partition, formed by the gills, sepa- 

 rates the suprabranchial chamber from the ventral mantle- 

 chamber throughout its entire length. In a few forms, such 

 as Cuspidaria, the gills become reduced to simple muscular 

 partitions perforated by pores and separating the two cham- 

 bers, practically all indication of the original ctenidium 

 characters having disappeared. 



The muscular system of the Pelecypoda reaches a some- 



