The gill slits at first open freely to the exterior, but during their formation two lateral folds of the body wall, containing a prolongation of the body cavity, make their appearance (fig. 4 A), and grow downwards over the gill clefts, and finally meet and coalesce along the ventral line, leaving a widish cavity between themselves and the body wall. Into this cavity, which is lined by epiblast, the gill clefts open (fig. 4 B, _br.c_). This cavity--which forms a true peribranchial cavity--is completely closed in front, but owing to the folds not uniting completely behind it remains in communication with the exterior by an opening known as the atrial or abdominal pore. The vascular system of Amphioxus appears at about the same time as the first visceral clefts. BIBLIOGRAPHY. (1) A. Kowalevsky. "Entwicklungsgeschichte des Amphioxus lanceolatus." _Mém. Acad. Impér. des Sciences de St Pétersbourg_, Series VII. Tom. xi. 1867. (2) A. Kowalevsky. "Weitere Studien über die Entwicklungsgeschichte des Amphioxus lanceolatus." _Archiv f. mikr. Anat._, Vol. XIII. 1877. (3) Leuckart u. Pagenstecher. "Untersuchungen über niedere Seethiere." _Müller's Archiv_, 1858. (4) Max Schultze. "Beobachtung junger Exemplare von Amphioxus." _Zeit. f. wiss. Zool._, Bd. III. 1851. (5) A. M. Marshall. "On the mode of Oviposition of Amphioxus." _Jour. of Anat. and Phys._, Vol. X. 1876. CHAPTER II. UROCHORDA[5]. [5] The following classification of the Urochorda is adopted in the present chapter. I. Caducichordata. {Solitaria _ex. Ascidia_. A. SIMPLICIA {Socialia _ex. Clavellina_. {Sedentaria _ex. Botryllus_. B. COMPOSITA {Natantia _ex. Pyrosoma_. {Salpidæ. C. CONSERTA {Doliolidæ. II. Perennichordata. _Ex. Appendicularia_. In the Solitaria, except Cynthia, the eggs are generally laid, and impregnation is effected sometimes before and sometimes after the eggs have left the atrial cavity. In Cynthia and most Caducichordata development takes place within the body of the parent, and in the Salpidæ a vascular connection is established between the parent and the single foetus, forming a structure physiologically comparable with the Mammalian placenta. Solitaria. The development of the Solitary Ascidians has been more fully studied than that of the other groups, and appears moreover to be the least modified. It has been to a great extent elucidated by the splendid researches of Kowalevsky (Nos. 18 and 20), whose statements have been in the main followed in the account below. Their truth seems to me to be established, in spite of the scepticism they have met with in some quarters, by the closeness of their correspondence with the developmental phenomena in Amphioxus.