The shell is deposited as to chitinous plates, which subsequently become calcified. It undergoes in the different genera great changes of form during its growth. With reference to the larval stages of other Articulata, a few points may be noted. The three-lobed larva of Terebratulina septentrionalis is provided with a special tuft of cilia at the apex of the front lobe. The arms appear to originate, in Terebratulina caput serpentis, as two processes at the sides of the mouth, on which the tentacles are formed. Provisional setæ do not appear to be formed in the lobed embryos of Thecidium and Terebratulina, but they appear at a later stage at the edge of the mantle in the latter form. The third lobe of Thecidium gives rise to the dorsal and ventral mantle lobes. Inarticulata. The youngest stages in the development of the Inarticulata are not known, and in the earliest stages observed the shell is already developed. The young larvæ with shells differ however from those of the Articulata in the fact that they are free-swimming, and that the peduncle is not developed. The larva of Discina radiata has been described by Fritz Müller (No. 331). It resembles generally a larva of the Articulata shortly after the tentacles have become developed. Five pairs of long provisional setæ are present, of which all but the hindermost are seated on the ventral lobe of the mantle. Shorter setæ are also lodged on the edge of the dorsal lobe. The mouth is placed on the ventral side of a protrusible oral lobe. It is imperfectly surrounded by four pairs of tentacles, which form a swimming apparatus. A fuller history of the development of Lingula has been recently supplied by Brooks (No. 325). The youngest larva is enveloped in two nearly similar plate-like valves, covering the two mantle lobes. The mouth is placed at the centre of a disc, attached to the dorsal valve, on the margin of which is a ring of ciliated tentacles. The general position of the disc and its relations may be gathered from fig. 138, which represents a diagrammatic longitudinal vertical section of the embryo. With the growth of the embryo the tentacles increase in number, the new pairs being always added between the odd dorsal tentacle and the next pair. There is an axial cavity in the tentacles which, unlike the cavity in the tentacles of the Polyzoa, does not communicate with the perivisceral cavity. As the tentacles increase in number, the lateral parts of the tentacular disc grow out into the two lateral arms of the adult, while the dorsal margin forms the median coiled arm. These changes are not effected till the larva has become fixed. The attachment of the larva was not observed; but the peduncle, of which there is no trace in the young stages, grows out as a simple prolongation of the hinder end of the body while the larva is still free. It had already reached a very great length in the youngest fixed larva observed. _Development of Organs._