\SCIDIACEA DEVELOPMENT OK THE FREE-SWIMMING LARVA. 365 



while the sub-chorda] diverticulum, which is directed backward (d), 

 yields, through simple growth, the other parts of the alimentary 

 canal (the oesophagus, the stomach and the intestine proper). This 

 diverticulum, as the larva lengthens, is said to form a eoil in which 

 we can distinguish a right descending portion, a ventral connecting 

 piece, and a left ascending part which ends blindly. The right 

 portion is said to give rise to the oesophagus, the connecting piece 

 to the stomach and the left ascending portion to the intestine (c/. 

 Fig. 168, '/and ed with Fig. 170, oe, m, and erf). The anus only 

 arises later, during larval life, by the blind end of the intestine be- 

 coming connected with one of the two so-called cloacal vesicles (that 

 on the left), these latter being ectodermal invaginations which will 

 be described further later. 



The account given by van Beneden and Julin (Xo. 10) differs 

 from that given above in so far as they derive only the descending 

 portion of the intestinal coil (consisting of the oesophagus and the 

 stomach) through direct growth from the posterior diverticulum 

 mentioned above, while they believe that the intestine proper arises 

 from the ventral surface of the dilated stomach as a secondary out- 

 growth. The point of origin of this secondary caecum, which is 

 directed to the left and upward, is said to lie somewhat far forward, 

 i.e., to be almost pre-cbordal. We shall see below (p. 521) that these 

 authors ascribe some significance to this observation. 



The oral apertun of the larva (which gives rise to the inhalent or 

 branchial aperture of the adult) is formed only shortly before the 

 larva hatches. The anterior pointed end of the alimentary canal, 

 just before reaching the sensory vesicle, bends dorsally. It there 

 comes into contact with an invagination which has arisen from a 

 thickened disc of ectoderm-cells. By the apposition of these two 

 structures and a breaking down or separation of the cells, the oral 

 aperture is formed (Figs. 167 and 168, p. 358). 



The emdostyle develops as a ciliated furrow in the antero-ventral 

 region of the branchial sac (pharynx), owing to the formation of two 

 lateral longitudinal swellings. We cannot here enter further into 

 the histological details of this structure, but must refer the reader 

 for these to the treatises of R. Hertwig, Fol and Seeliger (No. 

 50). We may, however, mention that this furrow is not purely 

 ventral in position, its anterior portion extending up towards the 

 dorsally placed oral aperture (Fig. 170, es). According to a recent 

 treatise by Willey (No. 54a), the rudiment, of the endostyle 

 originally lies in the most anterior part of the branchial sac in which 



