256 



THE OLD MOUTH AND THE NEW. 



ganglion, that lies between the appendage and the medullary plate. (Fig. 136.) 

 On the outer margin there may be a second row of sense organs, and various 

 infoldings of an excretory or respiratory nature. On its posterior basal surface a 

 gill, or respiratory plates or filaments, may be developed. 



Each appendage is associated with a hollow block of mesoderm, or somite, 

 that lies beneath the basal lobe, and gives rise to the associated muscular and excre- 

 tory tissues. (Fig. 134, 138, so.) The mesoderm extends beyond the appendages 

 as a thin double layer of cells, the lateral plates, from which the somatic and 



Fig. 157. — Diagrams illustrating the growth of organic films on a nutrient surface. The figures are intended 

 to show how association, and the time element involved in combined apical and bilateral growth of an organic film 

 on a plain, or spherical nutrient surface, automatically creates lines of unlike conditions that are coincident with 

 the lines of morphological and physiological specialization. A B and C, Successive stages in the growth of such a 

 film, showing the necessarily unlike character of the initial and terminal element (cephalic and caudal) , and of 

 the median and lateral ones, and that this unlikeness increases with the progress of growth. Z>, A still older 

 stage, in side view perspective. 



splanchnic tissues of that metamere, if any are present, arise, l.pl. The cavities 

 enclosed in the mesoblastic somites, so long as the somites retain their identity, 

 do not communicate with one another. On the other hand, the space between 

 the somatic and splanchnic layers of the successive lateral plates, is not divided 

 into separate compartments (thorax), or if it is (abdomen), they speedily break 

 down, forming a continuous coelomic chamber on each side of the body. The 

 third elements associated with the appendages are the outgrowths from the mid- 

 gut that form the so-called "liver lobes," or the enteric diverticula, or the gut 

 pouches. (Figs. 150, 154, 179, 180.) 



