xn PHYLUM MOLLUSCA 693 



I 



forward by suddenly withdrawing its foot and thus ejecting water 

 through the siphons. The only parasitic genus is Entovalva, 

 found in the gullet of a Holothurian. 



Pelecypoda are abundant both in fresh water and the sea ; the 

 marine forms are mainly littoral. None are pelagic or terrestrial. 

 They are very abundant in the fossil condition, occurring in all 

 formations from the Upper Cambrian upwards, and, owing to 

 their gregarious habits, frequently forming extensive deposits or 

 shell-beds. The oldest forms are all iso- or hetero-myarian ; the 

 monomyarian types (Pseudolamellibranchia) appear first in the 

 Carboniferous, and the Siphoniata not until the Triassic period. 

 The modern genus Area dates from the Upper Cambrian, and thus 

 furnishes as striking an example of a " persistent type " as some 

 of the Brachiopods. 



There seems to be little doubt that the Protobranchia, and 

 especially Nucula, exhibit the most primitive type of pelecypod 



SINUPALLIATE 

 EULAMELLIBRANCHIA 



INTEGRIPALLIATE 

 EULAMELLIBRANCHIA 



PSEUDO-LAMELLIBRANCHIA 



HETEROMYARIAN 

 FILIBRANCHIA 



ISOMYARIAN 

 FILIBRANCHIA 



PROTOBRANCHIA 



FIG. 606. Diagram illustrating the mutual relationships of thelPelecypoda. 



organisation, as indicated by the plume-like gills with separate 

 filaments, the simple kidneys, and the distinct cerebral and pleural 

 ganglia ; absence of concrescence is always a mark of low or 

 generalised organisation. The Filibranchia with imperfectly united 

 gill-filaments come next, and are divisible into two groups iso- 

 myarian with equal-sized adductors, and heteromyarian with more 

 or less atrophied anterior and proportionally enlarged posterior 

 adductor ; the latter group is to be looked upon as the more special- 

 ised, and leads to the Pseudolamellibranchia (monomyarian type) 

 in which the anterior adductor disappears completely in the adult, 

 while the posterior is immensely enlarged and assumes a central 

 position. Similarly, the isomyarian Filibranchia lead to the 

 Eulamellibranchia, which are equal-muscled, but have the gill- 

 filaments united into a complete basket-work. In the Eulamelli- 



