ANNELIDA 



329 



organs, for these are not formed in the germ bands until the 

 primitive kidneys have already separated from them (comp. 

 also p. 332). 



Like the primitive kidneys, other organs of the larva also 

 degenerate during its metamorphosis into the adult worm. 

 A musculature consisting of longitudinal and circular fibres, 

 which in the region of the mouth enlarges into a powerful 

 circular muscle, is found under the epidermis of the larva. 

 Between the muscle fibres Bergh finds spindle-like and 

 branched cells, which he takes to be of a nervous nature. 

 This entire larval skin is said by Bergh to be cast off in the 

 metamorphosis, and the whole body of the leech, with the 

 single exception of the mid-gut, arises from the so-called 

 trunk and head germs (Rumpf- und Kopfkeime) , of which 

 more will be said later. At this time the mouth closes. 

 The provisional pharynx of the larva is replaced by a per- 

 manent one. Details about these pi^ocesses will be given 

 in considering the formation of the organs. 



3. The Further Development of the Body; For- 

 mation of the Head and Trunk. 



A distinction between head and trunk was apparent even 

 in the Chsetopoda ; it was recognizable by the condition of 

 the mesoderm, and also probably found expression in the 

 m.ode in which the nervous system was formed. In the 

 Hirudinea this contrast is still more decided, for the funda- 

 ments of the nervous system of the head and trunk are 

 separate, and the so-called germ bands probably take no 

 part whatever in the formation of the head. According to 

 the investigations of Bergh, which to a certain degree con- 

 firm and extend the earlier discoveries of Leuckart and 

 Semper, there are two so-called head germs (Kopfkeime) in 

 addition to the germ bands, which we have already learned 

 about, and which are designated by Bergh as trunk germs 

 (Btivipfkeime) . These head germs, the origin of which 

 is still obscure, lie between the pharynx and epidermis as 

 two cell-masses, which become united by a connecting cord 



