254 W. Ji. Coe — Development of the 



scribed above, and have no sensory function whatever. On the 

 nature of the other nervous structures which Salensky describe^ the 

 pilidia here recorded throw no light whatever. In no case was I 

 able to demonstrate that any supposedly nervous elements did not 

 actually belong to the muscular system. In this respect my results 

 agree with those of others who have worked upon similar pelagic 

 embryos of other groups. 



The Larval Mesenchyme. 



The mesodermic structures of the pilidiura are of two sorts, but 

 the cells from which they arise are at first indistinguishable from 

 each other. They later become widely different and form the mus- 

 cular system, described below, and the larval mesenchyme. The 

 mesenchyme cells float freely about in the body-cavity and doubtless 

 secrete a small quantity of transparent, gelatinous substance which, 

 with a large amount of water, fills up the whole cavit}^ 



The cells of the mesenchyme have already been well described by 

 Metschnikoff (25), Salensky (28), C B. Wilson (32) and others. It 

 is from these cells that the musculature of the adult nemertean de- 

 velops. Hubrecht (16) states that in Desor's larva the adult nerv- 

 ous system is also derived from the larval mesenchyme. Burger, on 

 the other hand, feels confident that such is not actually the case, 

 but that in Desor's larva, as in the pilidium, the nervous system 

 arises directly from the ectoderm (5, p. 4*75). The development of 

 the muscle-cells of the pilidium from the undifferentiated amoeboid 

 cells of the mesoderm has recently been described by C. B. Wilson.* 



Some of the mesenchyme cells become collected in clusters near 

 the edge of the lappets and below the intestine. These often con- 

 tain a number of clear vacuoles the contents of which perhaps con- 

 tribute to the formation of the transparent, jelly-like substance 

 which fills the whole body. Others are equally conspicuous because 

 of their decidedly yellowish color. They are, on the whole, not very 

 different from the original mesoderm cells. They are oval or amoe- 

 boid in shape, and are much lai'ger than the muscle-cells. They 

 have little to do with the formation of the pilidium and are held in 

 reserve until the body of the young nemertean begins to develop. 



* It is doubtless due to a typographical error that Wilson (32, p. 20) states that 

 "the mesenchyme first appears as isolated cells derived from the edotferm, as ob- 

 served by Metschnikofi." Metschnikoff (25, p. 53) says that they almost certainly 

 arise from the entoderm. 



