Fig. 137. Two stages in the development of Argiope, shewing the folds of the mantle growing over the cephalic lobe. (After Kowalevsky.)
m. mantle fold; me. mesenteron; pd. peduncle; b. provisional setæ.
After swimming about for some time the larva becomes fixed by its hind lobe, and becomes gradually transformed into the adult. The hind lobe itself becomes the peduncle. After attachment the mantle lobes bend forward ([fig. 137] A, m), and enclose the cephalic lobe. The valves of the shell are formed on their outer surface as two delicate chitinous plates ([fig. 137] B). At a somewhat later stage the provisional bristles are thrown off, and are eventually replaced by permanent setæ round the edge of the mantle. The cephalic lobe becomes located in the dorsal valve of the shell, and the mouth is formed near the apex of the cephalic lobe immediately ventral to the eye-spots, by an epiblastic invagination. The permanent muscles are formed out of the muscles already present in the embryo.
Around the mouth there arises a ring of tentacles, very possibly derived from the ciliated ring visible in [fig. 136][132]. The ring of tentacles is placed obliquely, and the mouth is situated near its ventral side. The tentacles appear to form a post-oral circlet, like that of Phoronis (Actinotrocha): they gradually increase in number as the larva grows older.
Fig. 138. Diagram of a longitudinal vertical section of an advanced embryo of Lingula. (After Brooks.)
a. end of valves; b. thickened margin of mantle; c. mantle; d. dorsal median tentacle; e. lophophore; f. lip; g. mouth; h. mantle cavity; i. body cavity; k. wall of œsophagus; l. œsophagus; m. hepatic chamber of stomach; n. intestinal chamber of stomach; o. intestine; q. ventral ganglion; r. posterior muscle; s. dorsal valve of shell; t. ventral valve of shell.
Some of the later stages in the development of the Terebratulidæ have been made known to us by the observations of Morse (No. [328]‑[9]) on Terebratulina septentrionalis.