HABITS, ETC., OP OEREBBATULUS LAOTKUS. 



185 



that in Cerebratulus there is no nervous tissue in or 

 connected with the apical plate. This plate differs 

 from its morphological homologue in annelid larvge in being 

 made of a single layer of undifferentiated ectoderm instead 

 of containing two layers more or less modified. In the tro- 

 chophore the plate contains nervous tissue, which afterwards 

 develops into the supra-oesophageal ganglion of the adult ; 

 in the Cerebratulus pilidium the plate is thrown away with 

 the rest of the aboral ectoderm during metamorphosis, and 

 we find no trace of any nerves at all. 



The Cilia Rows. — These are rows of large cilia running 

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

 the lappets, and they are the chief organs of locomotion. 



We can distinguish ectoderm wall cells, cilia cells, and a 

 well-developed locomotor muscle system (fig. 75). 



The ectoderm cells are similar in structure to other ecto- 

 derm, but increase in thickness as they approach the cilia 

 cells, those on the aboral surface being thicker than the oral. 



Hence the upper border overhangs the under one, and acts 

 as an organ of protection to the cilia, as noted by Salensky 

 (45). 



The cilia cells are smaller than the wall cells, and are 

 arranged in three rows between the latter along the extreme 

 edge of the lappets. They are wedge-shaped or pyramidal, 

 with their apices turned inward. Their cytoplasm and nuclei 

 are similar to those of the apical plate, and they are also 

 covered on the exterior with a structureless membrane, 

 through which the cilia pass. The latter are distributed over 

 the entire surface of the three rows of cells, but show a ten- 

 dency to gather in bundles. On the inner surface of the cilia 

 cells, and between the wall cells, lie the locomotor muscles (fig. 

 77). These consist of a strong muscle band and of scattered 

 mesenchyme cells. The band is made up of stout parallel 

 fibres, which originate in the mesenchyme cells and termi- 

 nate in the cilia cells (fig. 79), It is continuous with the 

 circumoral muscle both anteriorly and posteriorly (fig. 77), 

 and stains deeply with hsematoxylin. It thus corresponds 



