MYXINOIDEA 51 



the Myxinoids is as yet but very incompletely known (Dean [106], 

 Price [335]). 



The Myxinoids are all marine, and are unknown as fossils. 



Family Mixinidae. Bdellostoma, J. Midler (Homea, Flein.), Fig. 15 ; 

 Pacific. Paramyxine, Dean; Pacific (Fig. 31). Myxine, L. ; Atlantic, 

 North Sea, Pacific (Fig. 15). 



Sub-Class 2. PETROMYZONTIA. 



The Lampreys possess the following chief distinguishing features 

 in their structure : — 



A wide ' tooth '-bearing sucker surrounds the mouth. Since, 

 dorsally, it develops between the stomodeum and the hypophysis, 

 the nostril (hypophysial aperture) is carried up and far back on. 

 the dorsal surface of the head (Fig. 24), and is not terminal. 

 The cartilaginous skull and neural arches are well developed ; but 

 it is in the presence of an elaborate branchial basket, surrounding 

 the gills and ' tongue,' and even enclosing the pericardium, that 

 they differ most from the hag-fish (Fig. 20). 



The gill-pouches are seven in number on each side, open sepa- 

 rately to the exterior, and communicate internally with a suboeso- 

 phageal tube (p. 34). In the larva, however, the gill-pouches 

 open, as in all vertebrates, directly into the pharynx (Figs. 35 

 and 37). During the metamorphosis this region becomes nipped 

 off behind ; whilst the oesophagus grows forward and above, so as to 

 open into the buccal cavity in front of the gill-slits. 



Other characters, such as the blind hypophysial sac, the simpler 

 and hollow structure of the brain, the separate course of the dorsal 

 and ventral roots of the spinal nerves, the normal structure of the 

 eyes, the presence of two semicircular canals in the ear and of a 

 well-developed lateral-line system of sense-organs, the closure of the 

 pericardium, the more complex structure of the kidney, the spiral 

 valve in the intestine, the subdivision of the dorsal fin, etc., have 

 been mentioned above. 



It is in the Ammocoete larva of Petromyzon that some of the 

 strongest evidence of the primitive character of the Cyclostomes is 

 found to occur (Balfour [29], Scott [393], Dohrn [116], von Kupffer 

 [275], Koltzoff [273], Schneider [389], etc.). 



The eggs are thin-shelled, small, with comparatively little yolk, 

 and undergo holoblastic cleavage. The blastopore becomes the 

 anus. The first three mesoblastic somites develop as outgrowths 

 of the archenteric wall, and contain a coelomic cavity which for a 

 time is in communication with the archenteron as in Amphioxus. 

 The embryo develops into a larva differing very considerably in 

 structure from the adult, and undergoes a pronounced metamor- 

 phosis to reach the perfect state. In this Ammocoete many 



