xii PHYLUM^ MOLLUSCA 711 



species must be mentioned: they are deposits of nacre formed 

 usually round encysted parasitic worms, either between the 

 mantle arid shell or in the soft parts. They are produced, amongst 

 other species, by the " Pearl-oyster " (Afeleagrina margaritifera) 

 and by the Pearl-mussel (Unio margaritifera). Some species, 

 such as the common boring Pholas, are phosphorescent. 



Most Pelecypoda are sluggish in habit, progressing only by slow 

 contractions of the foot, and some are permanently fixed during 

 adult life by the byssus, or are only able to change their position 

 after throwing off the byssus, which becomes replaced by a new 

 one. The Scallops, however, swim freely by clapping the valves 

 together. The Cockles (Cardium), Trigonia, &c., jump by sudden 

 movements of the foot, and the Razor-fish (Solen) jerks itself 

 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 

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

 filaments, the simple nephridia, 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 

 isomyarian 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 

 specialised, 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- 

 branchia, lastly, there is a gradual series of stages from compara- 

 tively generalised forms with free mantle- lobes up to the highly 

 specialised species with large siphons. That the Pseudolamelli- 

 branchia and the siphoniate Eulamellibranchia are to be looked 



