Before the four segments are established the whole embryo is covered with cilia[131], and two and then four rudimentary eyes are developed on the anterior segment of the body. [130] It should be stated that it is by no means clear from Kowalevsky's figures that he agrees with Lacaze Duthiers as to the succession of the segments. [131] Kowalevsky in his figures leaves the penultimate lobe unciliated. _The history of the Larva and the development of the organs of the Adult._ [FIG. 136. LARVA OF ARGIOPE. (From Gegenbaur, after Kowalevsky.) _m._ mantle; _b._ setæ; _d._ archenteron.] Articulata. The observations of Kowalevsky and Morse have given us a fairly complete history of the larval metamorphosis of some of the Articulata, while some of the later larval stages in the history of the Inarticulata have been made known to us from the researches of Fritz Müller, Brooks, etc. The embryo of Argiope, which may be taken as the type for the Articulata, was left (fig. 135 B) as a three lobed organism with a closed mesenteron and a body cavity divided into two lateral compartments. On the middle segment of the body dorsal and ventral folds, destined to form the mantle lobes, make their appearance, and on the latter two pairs of bundles of setæ are present (fig. 135 B). The setæ together with the mantle folds grow greatly, and the setæ resemble in appearance the provisional setæ of many Chætopods (fig. 152). On the hinder border of the mantle cilia make their appearance. The anterior or cephalic segment assumes a somewhat umbrella-like form, and round its edge is a circlet of long cilia, while elsewhere it is provided with a coating of short cilia. Two pairs of eyes also arise on its anterior surface (fig. 136). [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.