DEVELOPMENT OF AGARICIA. 



491 



and sixth pairs, when a difference can be observed between the two 

 sides of the mesenteries, the fibres are most numerous on the dorsal 

 side (Fig. 2, E). In the mesenteries in which a mesenterial filament 

 is developed (first and second pair), the muscle fibres are most 

 numerous in that region of the mesentery which is about twice as 

 far from the body wall as from the internal margin of the mesentery, 

 in those sections in which the filament appears. In all other cases 

 the muscle fibres are most numerous along the internal edges of the 



Figure 1. Agariciafragilis. Larva A. A , a longitudinal section through 

 the first pair of mesenteries and their filaments, reconstructed from transverse 

 sections. Viewed from the dorsal side. The numbers, and the dotted lines 

 corresponding to them, indicate the numbers and positions of the transverse 

 sections of the series. Ectoderm and mesoderm black. B, C, D, E, and F 

 represent sections number 14, 18, 23, 30 and 41, respectively. The reader 

 views the aboral face of the section, and dorsal is up, so that the right of the 

 larva is on the left of the figure. In C, J, II, III, IV indicate the first, second, 

 third and fourth pairs of mesenteries, respectively. X 90. 



mesenteries and are usually numerous on both sides of the mesentery 

 along this edge. 



The musculature of the body wall consists of a layer of longitudinal 

 fibres developed in the inner ends of the ectoderm cells where they 

 abut on the mesogloea and of a layer of circular muscles developed in 

 the endoderm cells where they reach the mesogloea of the body wall. 

 The mesoderm of the body wall, although thin, is everywhere clearly 

 to be seen. 



