HABITS, ETC., OF OEREBRATULUS LAOTEUS. 169 



sphincter, whose contraction can almost close the mouth 

 opening. These ring and band muscles form an excellent 

 support for the attachment of the other muscles, notably 

 those of the lappets and apical plate, and for the support of 

 the primitive intestine. In fact, they serve as a sort of 

 centre for the whole body musculature ; and this seems an 

 important use under normal conditions. They also restore 

 the oesophagus to its normal shape after dilation, and, next to 

 the apical muscle, are the most strongly constricted under 

 irritation. They have the advantage of being situated close 

 to the source of supply of mesenchyme cells, and in other 

 species they are retained during metamorphosis, and develop 

 into the strong lip muscles of the adult Nemertean. 



The second kind of mesenchyme given off by the entoderm 

 consists of much smaller cells than the first. Very soon 

 after the appearance of the large cells these smaller ones 

 may be seen coming off in the same locality. The difference 

 in size is shown in fig. 9, and is not diminished by subsequent 

 growth. These micromesencytes do not float about like 

 their larger predecessors, but remain close to the body- wall 

 (fig. 14). 



They do move about, however, in a curious way.^ 

 After being warped about in different directions they each 

 settle down at some particular spot, and become permanently 

 attached, to form the fifth group of muscles. Conn, in his 

 study of Thalassema (17), found two kinds of mesenchyme 

 cells, one of which became distinct muscles, while the other 

 consisted of branching cells of the same origin, but more like 

 " connective tissue corpuscles scattered at random in the 

 body-cavity, but quite close to the body- wall. In the older 

 larvae they are more abundant, and unite to form quite a 

 continuous layer." Their similarity in origin, form, and 

 position to the micromesencytes of Cerebratulus, and the 

 fact that they are ultimately joined in a continuous layer, 

 suggest that they may have a similar function. 



^ In a verbal communication Mrs. G. E. Andrews states that she has seen 

 these cells warping themselves about by means of protoplasmic processes. 



Ik 



