HABITS, ETC., OF CEREBRATULUS LACTEUS. 163 



At first the oesophagus is cylindrical and of the same dia- 

 meter throughout J but later it is enlarged laterally and con- 

 stricted around the mouth as well as at its junction with the 

 stomach. This dilation produces a lateral elongation of the 

 mouth opening, which becomes more and more elliptical. 



Both oesophagus and stomach walls are covered with 

 muscle-cells. In the former muscles are formed_, which, by 

 inflation and constriction, assist in taking in and ejecting the 

 food, and probably also in breathing. For in view of the 

 fact that the adult breathes through these same oesophagus 

 walls, and that a strong current of water is kept flowing 

 over them by the action of their cilia, it seems rational that 

 oxygenation should take place here in the larva. On the 

 stomach the muscles do not seem strong enough to render 

 much assistance in churning the food, but at the constrictions 

 they develop into strong sphincter muscles. The one around 

 the mouth in other species becomes the lip muscle of the 

 adult worm. 



The simplicity of this primitive intestine is the more re- 

 markable when we reflect that it is the only portion of the 

 pilidium which is preserved in the adult Nemertean. 



Mesoderm Formations. — These include mesenchyme 

 cells and the whole musculature of the larva. In no other 

 larva with which I am acquainted can the development of 

 the muscular system be followed so easily and in such detail. 



This is due to a combination of peculiarly favourable cir- 

 cumstances. First the pilidium walls are as clear as glass, 

 enabling the single mesenchyme cells to be seen through 

 them from the very beginning. Then the mesenchyme cells 

 themselves are large, and as soon as they become polar can 

 be readily distinguished from all others. Finally the develop- 

 ment into muscle is so gradual, and the ultimate tissue is so 

 simple, that a single cell may be followed for some time after 

 it has united with others, and has begun to function as 

 muscle. 



Consequently the development of the muscle system of the 

 pilidium larva is one which discloses something of the real 



