408 STUDIES IN EVOLUTION 



stage of Terebratalia and other genera of the Terebratellidse 

 it is called the gwyniform stage. 



By the time a length of 1 mm. is attained (Plate XXIV, 

 figure 12, and Plate XXV, figure 1) a triangular septum is 

 visible anterior to the middle of the dorsal valve. Its eleva- 

 tion inflects the circlet of tentacles, making the lophophore 

 reniform or bilobed, and producing a brachial structure 

 similar to that in Cistella. This stage is therefore called 

 the cistelliform stage. The only advance over this condition 

 shown in adult Cistella is that the band supporting the cirri 

 is calcified and attached to the crural points. In Terebra- 

 talia, during the growth immediately following and still 

 included in the cistelliform stage, the septum increases in 

 height, and often two small lateral expansions appear on the 

 posterior sloping edge (Plate XXV, figures 3, 4). 



In the transformation to the next, or platidiform, stage 

 (figures 5-8), the crural points appear, and there is a narrow 

 groove along the top of the septum, which is arched over at 

 the posterior end, forming at this point a small cylinder, 

 nearly vertical to the floor of the valve. This is the begin- 

 ning of the ascending branches, or secondary loop. The 

 septum is quadrangular, and develops on its anterior edge 

 several spinous processes (figure 7). The growth of the de- 

 scending branches from the crura and their union with the 

 septum bring about the platidiform stage, as represented in 

 Plate XXV, figure 8; and the growth of the ascending 

 branches produces the ismeniform structure (figure 9). The 

 expansions on the sides of the secondary loop are not present 

 in all specimens, and do not seem to be important characters. 

 The cirrated band at this time (Plate XXIV, figures 4, 5) 

 extends continuously from behind the mouth to the origin 

 of the descending branches, and along them to the septum, 

 thence obliquely backward on the septum, and over the 

 ascending branches to the median line, where new cirri are 

 introduced. 



The ascending lamellae from the septum already have begun 

 to divide or separate anteriorly, and in the next stage (Plate 



