HABITS, BTO.5 OF CBREBRATULUS LAOTEUS. 165 



tlie body-wall^ or tliat of the intestine^ to which they fasten 

 themselves. They then function as muscle by contracting 

 and pulling together the parts to which they are attached. 



While they remain free no trace of striation can be seen 

 in the cells^ but we have to remember that we are looking at 

 them through the pilidium wall^ which, however clear it may 

 be, is not perfectly diaphanous. Hence there might exist a 

 fine striation without our being able to detect it. But as 

 soon as the cells become fixed in position granules appear 

 in the cell cytoplasm, and the thread-like branches become 

 distinctly fibrillar in structure. No amitotic division stages 

 were found, as noted by Montgomery in the free mesenchyme 

 cells of the adult worm (37). Occasionally the branches of 

 two cells anastomose before becoming attached, and may be 

 seen to contract, at first irregularly, but later rhythmically, 

 like similar cells in opisthobranch Gastropods (cf. 49). After 

 the cell becomes fastened to the pilidium walls these rhythmic 

 pulsations cease, and the cells become regular muscles, con- 

 tracting only when stimulated. The muscles developed from 

 these macromesencytes are considered under the following 

 heads : 



1. Retractor Muscle of the Apical Plate. — This is 

 usually the first muscle formed. Soon after the intestine has 

 turned down posteriorly mesenchyme cells are seen along the 

 apex of the bend beneath the apical plate (figs. 8, 9). One 

 or more of them become stationary at about the position of 

 the future muscle. 



It then sends out spin-threads which reach, and fasten 

 themselves to, the apical plate, the wall of the intestine, and 

 sometimes the aboral wall of the pilidium. Other cells in 

 floating about come in contact with these fixed ones, and are 

 arrested. They in turn send out processes to the apical 

 plate, the intestine, or the wall of the pilidium, according 

 to their position. Other processes are then sent out which 

 anastomose with those already formed, making a coarse net- 

 work (fig. 11). 



The strands of the network are then pulled around by the 



