The digestive canal consists of three parts, the anterior 

 (the pharynx) , the middle (stomach) and the posterior 

 (intestines), all covered by cells with cilia, similar to 

 the cells of the skin. The digestive canal is attached to the 

 body cavity by fibrous tissue; it forms a network united with 

 the internal surface of the skin. In the junctions of the 

 network, nucleus-like formations are situated; they are 

 numerous also under the skin, in the neighborhood of the 

 calciferous skeleton. Krohn noted Johannes Miiller's error 

 when he assumed them to be strong fibers instead of nerves, 

 and the nuclei of fibrous tissue instead of nerve ganglia. 

 The larva swims with its limbs, mouth opening forward; its 

 movement is carried out by the activity of the cutaneous 

 twinkling cilia, which also drive into the mouth food particles 

 suspended in the water. 



The formation of the larva described here takes place as 

 follows. After the division is completed, the young spherical 

 larva rotates in the ovum membranes with the help of the 

 long cilia which cover all the surface of its body. In the 

 larva at this time can be observed a closed central cavity 

 and cover, which can be differentiated, forming the wall of 

 the latter. Krohn erroneously considered that the wall of 

 the body consists of many layers of cells. Within a day after 

 fertilization, when the number of the cover cells becomes 

 sufficiently large, the ovum membrane is torn and the larva 

 begin to swim. Soon after hatching it acquires its ovoid form 

 (Figure 45, A); during swimming its narrow end is directed 

 forward. 



In the body cavity of the larva, from the side of the 

 blunt end, an accumulation of closely situated dark bodies, 

 similar to the nuclei, is seen. The number of these nuclei 

 at first is small, but it quickly increases so that they are 

 found filling half of the body cavity of the larva, toward 

 its blunt end (Figure 45, A,e) . Later, nuclei are separated 

 from each other and distributed evenly in the body cavity, and 

 then partially accumulate not far from the rudiments of 

 the calciferous skeleton. After this, the nuclei become angular 

 or fusiform and begin their transformation into the fiber of 

 the reticular tissue which strings the body cavity and holds 

 the digestive tract. 



577 



