MOKPHOLOGY OF LAMELLlBRAiNCHlATE MOLLUSK.S. 4-25 



GENERAL CONSIDERATIONS. 



Change.s hi structure brought about by the degeneration of the foot. — It has been 

 demoiistrated that the aortse and sinus veiiosiis in Anodon possess valves or sphinc- 

 ters, wiiieh are supposed to be oi)erated at different times. If the foot is to be 

 expanded, the sphincter of tlie posterior aorta closes and all the blood is driven 

 through the anterior aorta into the foot. The supply conies into the heart at this 

 time tr()ni the mantle. Durin<i' this process, the valves of the sinus venosus are also 

 closed, and the blood is confined to the foot, which it extends. 



If the animal wishes to withdraw the foot between the valves of the shell, tlie 

 valves of the anterior aorta, which leads to the foot, are closed, and the sphincter of 

 the posterior vessel extending* to the mantle, as well as the valves of the sinus venosus, 

 is opened. The muscles of the foot, together with the retractor nuiscles, now contract 

 and force the blood into the sinus venosus, thence to the gills, heart, and then 

 through the only open aorta, the posterior one, into the mantle. 



As has been said, Ostrea has no foot, though its anoestcn-s nmst have possessed 

 one, and the mantle, being part of the mechanism used by many forms with a large 

 foot for its i)rotiusion and retraction, has correspondingly lost most of its great blood 

 spaces, which were capable of holding immense quantities of blood. The mantle has 

 become firm by the substitution of compact tissue and in Mytilus and other forms by 

 the reproductive glands, and its great blood supply has been reduced to the small 

 amount which the animal needs for the nourishment of its tissues. The posterior 

 aorta, having once snpplied the mantle inainly, is now distributed instead through the 

 greatly developed adductor muscle. 



I would point out the fact that there seems to be a correlation between the aborted 

 or absent foot and a thick mantle with no large blood spaces; and also between a 

 fully developed locomotor foot and a mantle consisting mainly of immense blood 

 s])aces. Pecteu maybe an exception to this rule, though I think it very probable that 

 large blood spaces do not exist in its thin mantle. This, of ccmrse, confirms the view 

 spoken of above, concerning the physiology oi' locomotion by the foot. It also shows 

 how the condition of some of the organs in Ostrea and MytiJus has been broTight 

 about by the degeneration of the foot possessed by a locomotor ancestor. 



The muscle system of Mytilus. — It will be noticed from the figures that the visceral 

 mass is relatively small and that a large part of it is filled by the great byssus mus- 

 cles. When the fixed habit was acquired and the animal fastened itself by means of 

 the byssus, these muscles Avere probal)ly developed in order to prevent a tearing of 

 the delicate tissues when the animal Avas subjected to the force of the waves. This 

 injury would easily have taken pla<;e if there had been no support to the byssus by 

 some attachment of it to the shell. Fig. 37 shows in section a jiowerfiil muscle on 

 either side, attached to the shell dorsally. These come together on the median line, 

 and their combined fibers run downward and become attached to the base of the 

 byssus organ. In Fig. 42, a side view of the Tnain nniscular system, it will be seen 

 that any downward pull from the byssus would bring a strain to l)ear tm all the 

 nuiscles whose outer ends are attached to the shell. 



Having been crowded out by the develoy)ment of such a bulk of nniscle tissue in 

 the visceral mass, it was necessary for tlu' generative gland to find another position. 



