STRUCTURE 



ganglion. No neural gland is found. The branchial aperture or 

 mouth leads into the simple branchial sac or pharynx (Fig. 30, 

 br.s). There are no tentacles. The endostyle is short, is a closed 

 tube both anteriorly and posteriorly (Fig. 29), and has about four 

 longitudinal rows of gland -cells. There is no dorsal lamina, 

 and the peripharyngeal bands run dorsally and posteriorly to 

 unite close in front of the oesophageal opening. The wall of the 

 branchial sac does not show the complex structure usual in Tuni- 

 cata, and has only two ciliated apertures (Figs. 30, 31, 32, sg}. 

 These are homologous with the primary stigmata of the typical 

 Ascidians, and with a pair of the gill -clefts of Vertebrates. 

 They are placed far 

 back on the ventral 

 surface, one on each 

 side of the middle 

 line, and lead into 

 short funnel-shaped 

 tubes which open on 

 the surface of the 

 body behind the 

 anus (Fig. 30, at}. 

 These tubes corre- 

 spond to the right 

 and left atrial in- 

 volutions, which in 

 an ordinary Asci- 

 dian fuse to form 

 the peribranchial 

 cavity. The remainder of the alimentary canal consists of 



cf 



' Transverse section through anterior part of Oiko- 

 pleura to show ganglion, sense-organs, endostyle, etc. 

 x 300. br.s, Branchial sac ; c.f, ciliated funnel ; ec, 

 dorsal ectoderm ; end, closed anterior end of endo- 

 style ; hy, hypobranchial groove in floor of branchial 

 sac ; n.g, nerve-ganglion ; or.gl, oral gland ; ot, otocyst ; 

 x, opening of ciliated funnel into pharynx. 



oesophagus, stomach (which may have a glandular diverticulum), 

 intestine and rectum (Fig. 30). The heart, surrounded ventrally 

 by a delicate pericardial membrane, lies below and in front of the 

 stomach, and is formed by the differentiation of the outer ends 

 of epithelial cells into muscular fibrillae. Two specially large 

 glandular cells are placed at the opposite ends of the heart. 

 There are no blood-vessels except the remains of the primary 

 body -cavity (blastocoel). No heart can be seen in some of the 

 smaller species of Oikopleura. Nearly all the species are herma- 

 phrodite, and the large ovary and testis are placed at the posterior 

 end of the body. There is no proper oviduct, the genital pro- 



