THE DEVELOPMENT OF ASTERINA GIBRBOSA. 383 
body-wall is illustrated in Pl. 28, 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 H, 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 thecells 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 amoebocytes or wandering cells (amed.), 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 pedicellariz, 
The superficial position of these muscles renders it exceedingly 
