282 SYNOPTIC COLLECTION. 



in (2). These two stages represent the protembryo. 

 The blastula or mesembryo is seen in (3), where there 

 are many cells around a central cavity. The gastrula or 

 metembryo (4) is formed in this genus by the turning in- 

 ward of a portion of the outer layer, a process known as 

 embolic invagination. In the first stage of the neoembryo 

 (5 ; also PI. 698, fig. i) the embryo consists of two seg- 

 ments, the cephalic and the caudal. Then the thoracic 

 segment develops (No. 697, 6 ; PI. 698, fig. 2) with the eye 

 spots and the four bunches of setae. A side view of this 

 cephalula stage is given in PI. 698, fig. 3, and a dorso- 

 ventral longitudinal view of the same stage in fig. 4. 

 The mantle lobes now cover most of the caudal segment, 

 and the cephalic segment is shaped like an umbrella (No. 

 697, 7). This is the completed neoembryo. The mantle 

 lobes fold upward over most of the head segment, and the 

 larva is transformed into the typembryo (see No. 697, 8 ; 

 PI. 698, fig. 5 ; fig. 6, longitudinal section of the same ; 

 No. 697, 9, the completed typembryo showing embryonic 

 shell). The mantle lobes are now directed forward in- 

 stead of backward, as in fig. 4, and the shell-secreting 

 surfaces (indicated by the heavy black line) are on the 

 outside instead of the inside. This stage develops into 

 the phylembryo (No. 697, 10; PI. 698, fig. 7); the latter 

 figure shows the protegulum, the beginning of the tenta- 

 cles on the band or lophophore (/), the hinge line, and 

 the teeth (/) at the outer ends. The nepionic stage is 

 represented in PI. 698, figs. 8-10. The tentacles are 

 now distinct (fig. 8), and also the mouth, stomach, and 

 muscles are shown with the shell and the pedicle. The 

 large opening between the valves (filled by the pedicle, 

 /) is well seen in figs. 9, 10, which are a dorsal and a 

 side view of the nepionic shell. 



The family Terebratellidae is especially instructive as 

 it offers interesting correlations. Beecher has brought 

 out (see PI. 699) the parallelism existing between adult, 

 permanent generic structures of the more primitive mem- 



