PLATE XIX. 



MANICINA AEEOLATA Linnseus. 



Fie;. 131. — Transverse section through the stomodseum of a polyp, showing the relationship of the ridges and furrows 

 tn the internal attachment of the mesenteries. X 110. 



Fig. 132. — Transverse section through a young polyp toward the inner termination of the stomodseum. The mesen- 

 terial plan is represented on p. 504, fig. 13/. At this level the edge-zone is already terminated at some 

 places, hut |»-i-i-i- at others. Onh certain of the perithecal continuations of the mesenteries extend 

 thus far, and some have ceased their connection with the skeletotrophic wall, while retaining that with 

 the column wall. The isolated portion of the column wall at the upper side of the figure is exception- 

 ally distended, and only fragments of the mesenteries are included. X 70. 



Fig. 133. — Freshly extruded larva?. In the one viewed from above CO three pairs of mesenteries are already united 

 with the stomodseum, and three other pairs are free. Enlarged. 



Fig. 134. — Another larva, about four days old, adherent to a plate of glass. The mesenteries are at almost the same 

 stage as in the larva represented in tig. 133 (/>), but pair 111, III, has not yet reached the stomodseum. 

 Enlarged. 



Fig. 135. — Young polyp, twenty-one days after extrusion, fixed to glass and viewed as a transparent object. The 

 tentacles are incapable of complete retraction, and appear as twelve spheroidal knobs arranged in two 

 alternating cycles of six each. The eight Edwardsian mesenteries are complete, but the remaining four 

 are incomplete. The skeleton is represented by six entoceelic radiating septa; the basal plate was also 

 developed, but is not shown. X 70. 



Fig. 136. — Another young polyp of the same age, in which the formation of the six septa has not proceeded 

 regularly. The first trace of the columella appears in the middle. The tentacles are not distinctly 

 situ, having become depressed in the discal wall. X 70. 



Fig. 137. — Vertical section of the young polyp represented in fig. 136, after decalcification. The section passes 

 through the wide oral aperture; the stomotUeum terminates freely on the left side, but is in union with 

 a mesentery on the right side. A tentacular thickening occurs on the left, and serves to delimit the oral 

 disk and column wall. The basal disk is practically devoid of any ectodermal (calicoblast) layer, but 

 toward the free column wall at each end it begins to appear. A septal invagination occurs on each side, 

 the polypal wall resting upon them. Histologically the endoderm of the basal disk differs from 

 that of the column and oral disk. X 200. 

 636 



