CH^TOPLEURA. 115 



Drew: The Habits and Movements of the Razor-shell Clam, Ensis direc- 



tus. Biol. Bui., 12, 1907. 

 : The Physiology of the Nervous System of the Razor-shell Clam, 



Ensis directus. Jour. Exp. Zool., 5, 1908. 



AMPHINEURA. 



CHAETOPLEURA. 



It will be profitable to study only external features, unless 

 time is to be had for cutting and studying sections, as the species 

 is small and difficult to dissect. Its apparently generalized struc- 

 ture, and its adaptations, make it desirable for students to under- 

 stand from descriptions and figures the main features of its an- 

 atomy. 



1. Examine specimens that are attached to stones and shells 

 and see how nicely they adapt their shapes to the shapes of the 

 objects to which they are attached. How is this possible? 



2. Remove a specimen and quickly transfer it to a clean 

 glass slide, applying its ventral side to the glass. Put your 

 finger in its back and prevent it from curling for a minute. It 

 will then generally remain attached to the slide and may be 

 studied from both sides. 



3. How many plates are there? What is the shape of each? 

 Do they apparently join edge to edge or do they overlap? Do 

 the plates extend clear to the margin of the animal? What 

 reason is there for having plates instead of a solid dorsal shell ? 



4. Notice the thickened margin of the animal, and see that 

 dorsally it bears spicules, while ventrally it is smooth and is 

 applied closely to the slide. 



5. Notice the flattened elliptical foot. Do you understand 

 how the animal creeps and adheres? 



6. In front of the foot is the head fold in which the mouth can 

 be seen. 



7. In the furrow bordering the foot are the gills. 



8. Remove the animal from the slide and see how it curls up, 

 Try to unroll it. 



