174 



OHAS. B. WILSON. 



6. Prior to fixation two macromesencytes sometimes anas- 

 tomose with tlie same change in structure and activity, but 

 they pulsate more or less rhythmically instead of contracting 

 in the ordinary manner. As soon as they fasten to the larval 

 walls, however, the pulsations cease, and their subsequent 

 contractions are like those of other muscles. 



7. The cytoplasm of the macromesencytes is exhausted in 

 the formation of branches. The nucleus retains its original 

 position until the muscle-fibres are formed; it may then 

 become flattened and incorporated in situ in the muscle 

 tissue, but more frequently it migrates along the fibre to the 

 pilidium wall. 



8. The micromesencytes do not float about, but remain 

 close to the pilidium walls, upon which they creep along to 

 their final position by means of protoplastic activities.^^ 



9. The ends of the thread-like pseudopodia sent out by 

 both kinds of mesenchyme, on coming in contact with a cell 

 of the pilidium wall, at first fuse with its ectosarc, but after 

 they become fibrillated the fibrillae are attached to and are 

 apparently continuous with the cell reticulum. 



10. The ectoderm and the entire musculature of the larva, 

 except the parietal muscles of the intestine and a few isolated 

 macromesencytes, disappear during metamorphosis. 



11. When irritated the larval muscles contract violently 

 and pull in pieces the walls of the pilidium, thus showing 

 the same tendency toward dismemberment that is manifested 

 in the adult. 



Conclusions. — The simple fact that circumstances com- 

 bine so favourably for a study of the musculature in the 

 pilidium would give it considerable interest. And when we 

 remember how closely it approaches in primitive simplicity 

 to the archetype of several of the largest groups of Inverte- 

 brates, it becomes doubly interesting, and we are enabled to 

 draw several suggestive conclusions. 



1. The muscles of the pilidium are metamor- 

 phosed pseudopodia. 



The pseudopodia of the mesenchyme cells are the exact 



