46 MYXINOIDEA 



In Petromyzon the anterior wall of the sinus is pierced by a 

 pair of genital apertures communicating with the abdominal coelom 

 (Fig. 28). In the Myxinoids, similar genital pores combine to open 

 between the anus and the kidney opening (Fig. 19). The genital 

 products in both sexes are shed into the coelom, and pass out 

 through these pores (Burne [74]) ; there are no other special ducts, 

 nor is there any communication between the testis and the kidney, 

 as in the Gnathostomes. The exact morphological significance of 

 the genital pores is unknown ; since, however, the genital ducts 

 may be occasionally reduced to very similar pores in the Teleostei 

 (p. 365), it is not impossible that these pores in the Cyclostomes 

 may be homologous with the Miillerian ducts of the Gnathostomes. 



The Cyclostomata are classed in two very clearly differentiated 

 Sub-Classes. No certain traces of fossil Cyclostomes have yet been 

 found. 



Sub-Class 1. MYXINOIDEA. 



The Hag-fishes, Myxinoidea, are distinguished from the Petro- 

 royzontia by the following chief characters : The single median 

 nostril (Fig. 29) is terminal, or slightly ventral, and the nasal canal, 

 strengthened by cartilaginous rings, is continued backwards below 

 the brain into a pituitary sac, which opens into the pharynx by a 

 secondary aperture pierced through in the late embryo (von Kupffer 

 [275]). On either side of the nostril and mouth are four tentacles, 

 supported by cartilages (Figs. 22, 23). They have plausibly been 

 compared to the oral tentacles of Amphioxus (Pollard [333]). 

 There is no toothed oral sucker, but a single large epidermal 

 * tooth ' is placed below the * ethmoid cartilage,' on the roof of the 

 buccal cavity (Fig. 22). The 'tongue' is more highly developed. 

 No neural arches are present in the trunk, the skull is more 

 membranous, and the visceral skeleton, except in front near the 

 skull, is reduced to mere vestiges near the external gill-openings 

 (Figs. 23, 27). Owing, apparently, to the excessive size of the 

 'tongue,' the gills and heart are pushed very far back (Fig. 31). 

 This migration of the branchial pouches behind the first three, which 

 disappear in situ, occurs somewhat late in development (Dean 

 [106]). Consequently the gill-openings are pierced between the 

 dorsal and ventral somatic muscles, irrespective of their metameric 

 order, when the gills reach their final position (the branchial nerves, 

 of course, follow the gills). There is always on the left side a 

 simple tube leading from the pharynx to the exterior, and open- 

 ing in common with the last gill-pouch it is the oesophageo- 

 cutaneous duct, probably a modified gill-slit (Figs. 23, 27). The 

 gills, in Bdellostoma, open independently to the outside, and there 

 may be as many as fourteen pairs. But in Myxine, where there 



