HABITS, ETC., OF CKREBRATULUS LAOTETJS. 189 



2. The ectoderm is a one-layered pavement epitlielium, 

 made up of cubical cells on the aboral surface and cylindrical 

 cells on the oral surface. The entoderm is also one-layered 

 and made up of cylindrical cells, but it is thicker and more 

 opaque than the ectoderm, and its cells overlap considerably. 



3. The aboral ectoderm remains constant after being- 

 formed, and the increase in size is accomplished by a strong 

 flatteniug without cell division. This, together v^rith a 

 gradual clearing of the cell protoplasm, gives the pilidium 

 great transparency. 



4. The apical plate is an invaginated thickening of the 

 ectoderm, made up of conical cells whose bases turn inward 

 toward tl^e body-cavity. The nuclei are pointed and some- 

 times prolonged into thread-like processes on the outer ends, 

 but are rounded and plump on the inner ends. The com- 

 ponent fibrils of the apical muscle connect with the inner 

 ends of all cells near the centre of the plate. 



5. The long stout cilia which form the apical tuft arise in 

 bunches of four to six from the centre of the inner ends of 

 the cells, and in life are gathered into a central bunch which 

 looks like a single large flagellum. 



6. The cilia cells, whence arise the large cilia running 

 along the border of the oral surface and around the edge of 

 the lappets, are arranged in three rows. The cilia them- 

 selves are given off in bunches from the outer ends of the 

 cells. 



7. Just inside the cilia cells is a strong band of locomotor 

 muscle-fibres and scattered mesenchyme cells. A. careful 

 study of their origin and development, the development of 

 the plate itself, its histological structure and staining, all go 

 to prove that both fibres and cells are muscle, and that there 

 is no nervous tissue in the plate or connected with it. 



8. The anterior wall of the oesophagus becomes partially 

 flattened and cleared in the same manner as the aboral 

 ectoderm, but the posterior wall retains its original thick- 

 ness. It is grooved along the mid-line to form a channel for 

 the food. 



