578 PHYLUM CHORDATA I CLASS CYCLOSTOMATA 



gullet, into the six pairs of respiratory pouches and their 

 efferent tubes, and leaves the body by the single aperture 

 at each side. The respiratory pouches have much- 

 plaited internal walls, on which the blood vessels are 

 spread out. On the left side, behind the sixth pouch, a 

 tube (the oesophago-cutaneous duct) opens from the 

 oesophagus to the exhalant aperture. Perhaps some water 

 enters by it in inspiration. 



Vascular system. — The blood contains the usual amoe- 

 boid leucocytes and red blood corpuscles, elliptical in 

 form (circular in the lamprey). It is collected from the 

 body in anterior and posterior cardinals, passes through 

 a sinus venosus into the auricle of the heart, thence to 

 the ventricle, thence along a ventral aorta, which gives off 

 vessels to the respiratory pouches. From these the purified 

 blood passes dorsalwards in efferent branchial vessels, 

 which unite posteriorly to form the dorsal aorta, while from 

 the most anterior a branch goes to the head. The portal 

 vein has a contractile sinus which drives blood through the 

 liver. The pericardium is in free communication with the 

 general body cavity. 



Excretory system. — The segmental pronephric ducts persist, 

 and give ofi short lateral tubules, metamerically arranged, ending in 

 globular Malpighian capsules. The pronephros is functional in the 

 young form, and at least part of it persists throughout life, e.g. in a 

 lymphoid structure beside the pericardium. 



The ducts end by separate pores on a papilla within the integu- 

 mentary cloaca. 



Reproductive system. — Myxine is a protandrous herma- 

 phrodite, spermatozoa being formed at an early period, 

 and ova afterwards. The reproductive organ is simple, 

 unpaired, and moored by a median dorsal fold of peri- 

 toneum. Owing to the large size of the ova, the ovary is 

 very conspicuous in full-grown forms. When the ova are 

 freed from the ovary, they pass into the body cavity. Each 

 has an oval horny membrane, with a circlet of knobbed 

 processes at each end. By these they become entangled 

 together. There are no genital ducts, but just above the 

 anus there is a large genital pore opening from the body 

 cavity into the integumentary cloaca. The development 

 is still unknown. 



