436 THE ANIMAL KINGDOM. 



surfaces of the pouches carry a series of longitudinal vascular folds, and in 

 the Lampreys tracts of the branchial epithelium are ciliated. 



The heart has the usual piscine structure, a sinus venosus, auricle and 

 ventricle. The bulbus aortae is small, the ventral aorta long, and gives 

 off as many branchial arteries as there are branchial sacs. Its anterior 

 part is deeply cleft, so that the three or four anterior branchial arteries of 

 either side come off from a right and left common vessel. The sub- 

 intestinal vein of the embryo persists in the adult, and is distributed to the 

 liver forming the portal vein. Its anterior part is said to be pulsatile in 

 Myxine. The blood corpuscles of the Lamprey are circular and nucleated. 



The segmental duct persists, and is not divided into Miillerian and 

 Wolffian ducts. It has a coelomic opening in the embryo Ammocoetes, 

 afterwards closed or lost. The kidney in Ammocoetes consists at first solely 

 of a pronephros with funnels opening into the pericardial region of the 

 coelome. This pronephros is afterwards aborted, whilst the anterior part of 

 the mesonephros becomes functional. It is said to be aborted in its turn at 

 the metamorphosis, a posterior series of segmental tubes being developed 

 at the same time. The pronephros of Myxine persists, and lies in the 

 pericardial cavity into which its tubes open. It is cut off, together with the 

 anterior part of the segmental duct, from the mesonephros. This latter 

 consists of the remainder of the segmental duct and a series of short simple 

 segmental tubes, one to each segment. The segmental ducts of the adult 

 open into a urogenital sinus, into which the coelome also opens by a pair 

 of abdominal pores. The sinus in the Lamprey communicates with the 

 exterior by an aperture placed on the apex of a papilla. The papilla lies 

 behind the rectal opening, and is inclosed together with it by a right and 

 left fold of the skin. The arrangement is essentially the same in Myxine 

 (Ewart). In the Ammocoetes the segmental ducts open into the rectum, 

 but the separation of the urogenital sinus from the rectum, and formation 

 of abdominal pores take place just before metamorphosis. The male and 

 female genital glands are unpaired. In Myxine the suspensory duplicature 

 of peritoneum is broad. The glands in Petromyzon are lobed. Both ova 

 and spermatozoa are shed into the coelome, and pass outwards through the 

 abdominal pores and urogenital sinus. The ova are impregnated ex- 

 ternally to the body. The ovum of Petromyzon Planeri is invested by an 

 adhesive mucous coat and a membrane composed of an inner perforated, 

 and an outer structureless layer, and there is a micropyle. Segmentation 

 is total but unequal ; and the archenteron is formed by invagination. The 

 epiblast cells grow over the larger yolk cells. The ovum of Myxine is 

 inclosed in an elliptical horny case with processes at each pole. Each 

 process ends in a three-armed anchor, by which the ova adhere one to 

 another in strings, and are probably attached to sea-weed. 



The Myxinoidei afford the only instance of parasitism among Verte- 



