PLATHELMINTHES 165 



then lie under the ectoderm like a calotte. According to 

 Lang's description also, there is formed from the four groups 

 of mesoderm cells a continuous layer, which attains to a 

 greater extension on the ventral side than on the dorsal 

 (Fig. 76 C). Only later does the mesoderm give rise to the 

 musculature of the body-wall and the connective-tissue 

 reticulum. By the fonnation of vertical mesodermal septa, 

 which advance from the periphery towards the median plane, 

 the branches of the intestine increase in length at the 

 expense of the central yolk-niass. New septa, which 

 encroach on these branches from the margin of the body, 

 split them up into secondary branches, so that the intestine 

 increases the number of its ramifications. 



When, finally, the greater part of the food-yolk has been 

 consumed, and the previously ellipsoidal embryo has under- 

 gone a flattening in the dorso-ventral direction, it breaks 

 through the egg-membrane and reaches the outside world 

 as a young Turbellainan. 



B. Indirect Development. 



The embryonic development takes place in a manner 

 similar to that of the forms without metamorphosis. There- 

 fore even in what has preceded we might, in a number of 

 instances, have considered forms with indirect development. 

 But, instead of developing into turbellarian-like forms, the 

 ovate embryo of this type acquires lobe-like processes (Fig. 

 77). They arise first by an elongation of the ectodermal 

 cells at the points affected, and then by an evagination of 

 the ectoderm. The typical larval form of the Turbellaria, 

 which arises in this way, is represented by Müller's larva, 

 as it has been named after its discoverer (Nos. 17 and 

 18). This larva (Fig. 77) possesses eight processes, three of 

 which are situated in the region of the mouth, two others 

 laterally, and three dorsally. They are provided at their 

 margins with a border of longer cilia. If these ciliated 

 appendages are to be compai'ed to the ciliated bands of other 

 larvae, they would have to be designated as the preoral 

 ciliated band, by means of which an oi^al area is separated 



