ANNELIDA 331 



the Nemertini and Hirudinea in a manner that is too inharmonious to 

 warrant a comparison.^ Also the further development of the "head 

 germs and trunk germs," which in the Nemertini takes place by means of 

 ectodermal invaginations and additions of mesenchyma cells, but in the 

 Hirudinea as early differentiations of embryonal cells, shows little 

 similarity, apart from the fact that the Annelidan and Nemertean larvae 

 themselves have only a very slight resemblance to each other. 



4. Formation of the Organs. 



The Body -covering. — At an early stage of embryonic de- 

 velopment the layer of small cells grows over the germ bands 

 and macromeres, and thus forms the epidermis. This epi- 

 dermis, beneath which muscles have already been developed, 

 probably from the germ bands, becomes in Clepsine the epi- 

 dermis of the adult worm, whereas in the Gnathobdellidas 

 it, together with its musculature, is said gradually to dis- 

 integrate, and to be replaced by a new epithelium, which is 

 formed from the superficial layer of the " head germs and 

 trunk germs." These have united in the region of the 

 mouth, and thus the entire body is covered by the new epi- 

 dermis. At the same time the body musculature is formed 

 from the head germs and trunk germs. The remnants of 

 the larval skin are finally cast off. 



According to the description given by Whitman for Clepsine, and by 

 Bergh for Aulastoma and Nephelis, the epidermis does not appear to be 

 homologous in the two groups, which differ from each other to the extent 

 that the larval skin in one group is directly transmitted to the adult 

 animal, but in the other is cast off, being replaced by a layer of different 

 origin. However, an intermediate condition is said to exist in Clepsine, 

 for, according to Whitman, two cells of the germ bands take part in the 

 formation of the epidermis, though Whitman explicitly denies that it 

 arises from these alone. 



In Clepsine there is developed out of the epidermis, between it and the 

 ganglion underlying the pharynx, a mass composed of numerous large 

 gland-cells, whose secretion serves to attach to the mother the newly 

 hatched young until their suckers are developed (Whitman). 



The Nervous System. — In the development of the Hirudinea 

 it is difficult to separate ectodermal and mesodermal ele- 



1 It should be added that Bergh himself afterwards ceased to place 

 any value on this quite natural comparison. 



