THE DEVELOPMENT OP ASTEEINA GIBBOSA. 265 



body-wall is illustrated in PI. XXI, figs. 146 and 147. These 

 sections are taken from young adults in which R equals '4 mm. 

 and -86 mm. respectively, and they pass through the same 

 region as fig. 145^ which is from a larva in Stage E, and which 

 we have already described. In fig. 146 we see that the mus- 

 cular fibres of the muscle we may call the dilator ani are 

 still connected with the peritoneal cells ; but in fig. 147 they 

 have become quite distinct, and the cells of the peritoneum 

 have become quite flattened. The ectoderm has entirely 

 changed its character, the numerous larval goblet cells have 

 almost disappeared, and the cells in general have become shorter; 

 many of them are inversely wedge-shaped, and are apparently 

 about to become converted into gland cells, probably of the 

 same histological character as those of the aboral wall of the 

 stomach. Here and there is a narrow cell ending in a fine 

 hair, one of the scattered sense-cells of the aboral surface; 

 these are shown in fig. 148, a piece of ectoderm from another 

 individual of the same age. All observers agree in maintain- 

 ing that the ectoderm of the adult retains its ciliated covering; 

 but though I have been able to make out easily the cilia, or rather 

 flagella of the metamorphosing larva, I have not been able to 

 do so with any certainty in the aboral wall of these young 

 adults. Probably the cilia are very delicate and fragile. The 

 tissues of the mesenchyme have undergone marked differentia- 

 tion. So far as my researches have extended it seems that 

 three fates are open to mesenchyme cells, all of which are 

 illustrated in fig. 147. They may remain practically unchanged 

 as amcebocytes or wandering cells (amoeb.), or they may become 

 embedded in bundles of fibres so as to form connective-tissue 

 cells (the fibres being intercellular, not outgrowths of cells) ; 

 or finally, they may fuse to form a syncytium having the form 

 of a meshwork (calc). This is the skeletogenous tissue; lime 

 is deposited in the interstices of the meshwork. There is a 

 fourth fate, which is not reached by any as far as I have gone, 

 but which obviously must be the lot of some, and that is to 

 form the muscles moving the spines or rudimentary pedicellarise. 

 The superficial position of these muscles renders it exceedingly 



