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. [132] In the abstract in Hoffman and Schwalbe Kowalevsky is made to state that the tentacles spring from the border of the mantle. This can hardly be a correct account of what he states, since it does not fit in with the adult anatomy of the parts. The figures he gives might lead to the supposition that they sprang from the edge of the cephalic lobe, or perhaps from the dorsal lobe of the mantle. [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 oesophagus; _l._ oesophagus; _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. The most interesting point in Morse's observations on the later stages is the description of the gradual conversion of the disc bearing the circlet of tentacles into the arms of the adult. The tentacles, six in number, first form a ring round the edge of a disc springing from the dorsal lobe of the mantle; in their centre is the mouth. In the later stages calcareous spicula become developed on the tentacles. When the embryo is far advanced the tentacles begin to assume a horse-shoe arrangement, which bears a striking, though probably accidental, resemblance to that of the tentacles on the lophophore of the fresh-water Polyzoa. The disc bearing the tentacles is prolonged anteriorly into two processes, the free ends of the future arms. By this change of shape in the disc the tentacles form two rows, one on the anterior and one on the posterior border of the disc, and eventually become the cirri of the arms. The mouth is placed between the two rows of tentacles, where the two arms of the lophophore meet behind. The position of the mouth was the original centre of the ring of tentacles before they became pulled out into a horse-shoe form. In front of the mouth is a lip. The arms grow greatly in length in the adult Terebratulina. In Thecidium the oral disc retains the horse-shoe form, while in Argiope the embryonic circular arrangement of the tentacles is only interfered with by the appearance of marginal sinuations.