CEPHALOCHORDA. 



439 



constricted by a muscle. Its aperture represents the original larval mouth. 

 The pharynx extends for nearly half the length of the body, and is traversed 

 from end to end by a ventral hypopharyngeal groove, the homologue of the 

 endostyle in Urochorda^ and, like it, furnished with ciliated cells. Its walls 

 are pierced by oblique branchial slits, more than 100 in number, in fully 

 grown individuals. The slits open externally into an atrial or peribranchial 

 cavity. The bars that separate them are alternately complete and incom- 

 plete, and in the latter case free at their ventral ends. The slits are 

 consequently U-shaped. The bars are supported by elastic rods, all con- 

 nected at their dorsal ends, and their inner or pharyngeal surface is covered 

 by a ciliated epithelium. The peribranchial cavity originates at an early 

 period by the growth of a right and left fold, or epipleure, from the dorsal 

 region, which meet and fuse ventrally, leaving a ventrally-placed abdominal 

 pore some little distance in front of the anus. The epipleures form two 

 prominent longitudinal folds the metapleures one on either side the 

 median ventral line. These folds, therefore, border a ventral furrow ; 

 but at the time, when the genital products are ripe, the consequent 

 distension of the epipleures smooths them away. The peribranchial cavity 

 on each side extends behind the abdominal pore for a short distance, and 

 the two cavities are continuous beneath the pharynx from side to side. 

 The epipleures contain the genital glands on their inner surface in the slight 

 extension of the coelome which they inclose. Their outer walls are mus- 

 cular, and, as before stated, the muscles are divided into myomeres. The 

 water used in respiration and the genital products escape by the abdominal 

 pore, but the latter not invariably. The pharynx is constricted posteriorly 

 into an ' oesophagus,' followed by a widening, the stomach. The intestine 

 runs straight to the anus, which lies asymmetrically on the left side. The 

 anal aperture has a sphincter muscle. A liver caecum opens into the 

 stomach, generally on the right, more rarely on the left, side, and projects 

 into the peribranchial cavity. Its lining cells, as well as those of the 

 stomach, contain a green pigment. The vascular system consists of a 

 dorsal aorta, double in the region of the pharynx, single behind it. The 

 sub-intestinal veins unite to form a vena porta, which supplies the walls of 

 the liver-caecum. The vein bringing away the blood from the caecum 

 runs ventrally beneath the pharynx, and sends up to the dorsal aortae, but 

 only along the complete branchial bars, branchial vessels, which are pro- 

 vided with small pulsatile dilatations at their ventral ends. Anteriorly to 

 the pharynx the ventral vessel is dilated and sinuous. It gives off a right 

 and left branch to the velum and a wide vessel on the right side, which 

 enters the right aortic trunk. The corresponding vessel on the left side is 

 seemingly rudimentary. The whoje system is tubular, and the portal vein 

 and ventral pharyngeal vessel are said to be contractile. The blood 

 corpuscles are for the most part amoeboid leucocytes. Rohon states that 



