LAMELLIBRANCHIATA 583 



relations are probably the most primitive of living lamellibranchs. 

 The specialization of the labial palps has had as its consequence the 

 partial suppression of the ctenidia, which remain in an undeveloped 

 condition. In this respect the Protobranchiata can hardly be held to 

 resemble the ancestral lamellibranch. 



Order FILIBRANCHIATA 



Mytilus (Fig. 393). While the majority of lamellibranchs are semi- 

 sedentary, the sea mussel has developed the sedentary tendency and 

 marks a half-way stage to the oyster which remains fixed through 



Fig. 398. Nervous system of A, Chiton, B, a lamellibranch. Dorsal views. 

 The outline of the mantle edge is indicated by a dotted line, buc.c. buccal 

 commissure and ganglia; ce.c. cerebral commissure; ce.pl. g. cerebro-pleural 

 ganglion; pal.n. pallial nerve; ped.g., ped.n. pedal ganglion and nerve; p.v.c. 

 palliovisceral commissure; sb.rad.c. subradula commissure; vis.g. visceral 

 ganglion. 



adult life. The mussel lives in association in beds between tidemarks 

 where the conditions are favourable. The very extensible foot is 

 tongue-like in shape with a groove on the ventral surface which is 

 continuous with the byssus pit posteriorly. In this a viscous secretion 

 is poured out which enters the groove and hardens gradually when it 

 comes into contact with sea water. The tip of the foot is pressed 

 against the surface to which the muSsel attaches itself, and in a cup- 

 like hollow which ends the groove the attachment plate is formed at 

 the end of the byssal thread. When one byssal thread has been formed 

 the foot changes its position and secretes another thread in another 

 place. The byssus thus consists of a mass of diverging threads arising 



