110 SUB-CLASS MARSIPOBRANCHII (CYCLOSTOMATA). 



The eye in Myxine is without any trace of lens and appears to have but 

 little if any pigment.* It consists of little more than the much degener- 

 ated optic cup. According to Miiller the optic nerve in Myxine passes 

 dorsal to the ophthalmic branch of the fifth. 



The auditory organ differs from that of other Vertebrata. 

 In Petromyzontidae it consists of a vestibule and two semi-circular 

 canals, in Myxinidae of a single semicircular canal only. 



The body cavity is divided into two parts, the pericardial 

 cavity and the general body cavity. These two cavities com- 

 municate by a wide opening on the right hand side in Myxinidae ; 

 in Petromyzontidae they communicate in the larva, but not in 

 the adult. The general body cavity opens into the urinogenital 

 sinus by two genital pores (one in Myxine}, through which 

 the generative products escape. 



In the anterior part of the body there is a system of spacious 

 venous sinuses. These are specially developed round the 

 branchial sacs, and in Myxinoids round the ventral aorta, thus 

 constituting a kind of haemocoelic body cavity for these parts. 



The vascular system is arranged essentially in the manner 

 usually found in fishes. The heart consists of sinus venosus, 

 large auricle, and ventricle. There are only two valves at the 

 junction of the ventricle and ventral aorta, and the base of the 

 ventral aorta (bulbus) is much swollen owing to the large amount 

 of elastic tissue in its walls. This bulbus is without any muscular 

 tissue. There is no conus arteriosus. The branches of the 

 ventral aorta are distributed one to each branchial sac, and the 

 efferent branchial vessels are collected into two aortic roots 

 which are continued forward as carotids, and unite behind to 

 form the dorsal aorta. The dorsal aorta is also continued 

 forward in the middle line as a carotid. In Myxinidae the 

 genital vein and some veins from the anterior part of the body 

 wall fall into the portal. Moreover, in Myxinidae the portal 

 vein is dilated into a contractile sinus, which, contracting about 

 as rapidly as the heart, drives the blood through the liver. It 

 is a remarkable fact that no muscular fibres can be found in the 

 walls of this portal sinus (J. Miiller). The portal vein extends 

 for some distance in the intestinal wall and has been called the 

 subintestinal vein. 



* C. Kohl, " Rudimentare Wirbelthieraugen," Bibl. Zool. (Leuckart & 

 Chun.), 4, heft 13, 1892. 



