5o 



MYXINOIDEA 



In addition, there is a supra-intestinal vein, which, joining an 

 anterior vein derived from the right cardinal, swells into a pulsatile 

 portal heart used for pumping blood into the liver (Figs. 27, 32). 

 The liver is subdivided into two separate lobes. The intestine 

 shows no spiral valve. 



The spacious pericardium surrounds the oesophagus, and remains 

 in open communication with the abdominal coelom on the right 

 side (Fig. 27). The tubules of the kidney are segment-ally 

 arranged (p. 87, Fig. 32). 



The brain differs considerably in shape, and its ventricles are 

 much reduced (Holm [217], Worthington [507a]. The vagus root 



has apparently fused with the glosso- 

 A B pharyngeal. The dorsal roots of 



772- aa the spinal nerves join the ventral 



roots. The small degenerate eyes 

 are sunk far below the skin ; both 

 the eye-muscles and their nerves 

 disappear (Figs. 21, 22). Only 

 one semicircular canal is present 

 in the ear, probably representing 

 the two vertical canals fused to- 

 gether, since there are two ampullae 

 (Fig. 33). 



Quite recently the lateral-line 

 system of Bdcllostoma has been de- 

 scribed by Ayers and Worthington 

 [26] ; it is in a very undeveloped 

 condition, and consists of groups 

 of sensory cells lodged in the epidermis of the head. The cavities 

 which overlie the sense-organs are closed, and only shallow grooves 

 indicate their presence on the outside. 



The extraordinary sliminess of the skin is principally due to the 

 activity of two longitudinal rows of large glandular sacs. These 

 are segmental, paired, lateral invaginations of the skin (Figs. 29, 

 32), in which are stored mucous cells (Blomfield [37a]). They are 

 of two kinds : granular mucous cells, and thread cells of very 

 remarkable structure, which may perhaps be homologous with the 

 club-cells in the skin of the Lamprey (Fig. 36). 



The median fin is not subdivided, and is not provided with 

 special muscles. 



Alone among Craniate vertebrates the Myxinoids are normally 

 hermaphrodite. They lay eggs of large size, richly provided with 

 yolk, and enclosed in elaborately finished shells of horny consistency 

 (Fig. 91). These shells are secreted in the ovary itself, and are not 

 homologous with the very similar egg-cases of Elasmobranchs. 



Cleavage is meroblastic ; but, unfortunately, the development of 



Pig. 33. 



Auditory labyrinth of Myxine glutinosa, 

 L. A, inner dorsal view ; B, inner ventral 

 view. (After Retzius, from Gegenbaur, 

 Vergl. Auut. Wirbeltiere.) a.a, anterior 

 ampulla ; a.p, posterior ampulla ; m.e, 

 macula communis ; r.a and r.p, ramus 

 anterior and posterior of auditory nerve ; 

 s.r, sinus endolymphaticus. 



