DEVELOPMENT OF LARVA 



585 



The changes at metamorphosis are very great. There are first 

 skeletal changes, the cartilaginous parts of the skeleton developing. 

 Then the suctorial mouth develops. Behind the last of the gill- 

 pouches the oesophagus grows forward 

 dorsal to the gill region and opens into 

 the top of the pharynx. Thus the 

 respiratory tube comes to lie below the 

 ahmentary canal, appearing as a back- 

 wardly projecting blind diverticulum of 

 the pharyngeal floor. Each gill -pouch 

 becomes separated from the tube by a 

 short duct, and communicates with the 

 exterior by a similar duct. The endo- 

 style shortens up and finally becomes 

 separated off from the pharynx as a 

 closed pocket — the thyroid gland. Other 

 changes include the disappearance of 

 the pronephros, the emergence of the 

 eyes, and the enlargement of the fin, 

 together with its division into the adult 

 state. 



Lampreys are distributed in the rivers 

 and seas of north and south temperate 

 regions. They are often used as food. 

 Besides Petromyzon there are several 

 related genera, e.g. Mordacia and Geotria, 

 from the coasts of Chili and Australia, 

 and Ichthyomyzon, from the west coast 

 of N. America. Certain structures 

 called " conodonts," from very ancient 

 (Silurian) strata, have been interpreted 

 as teeth of lampreys or hags. 



Incert^ Sedis J, 



Palaeospondylus gunni. — 

 Under this title Dr. Traquair 

 has described a very remarkable '^ 

 fossil form from the Old Red 

 Sandstone of Caithness. He 

 speaks of it as a " strange relic 

 of early vertebrate life." 



It is a dainty little creature, 

 somewhat tadpole-like at first 

 sight, usually under an inch in 

 length. The following characters 

 affinities with Cyclostomata : — 



IG. 329. — Restored skeleton 

 of PalcBospondylus gunni. 

 — After Traquair. 



.c, Cirri of dorsal margin ; /.(., 

 long lateral cirri ; v.c, cirri of 

 ventral margin ; n., nasal ring ; 

 t.p., anterior trabeculo-palatine 

 part of cranium ; b., anterior 

 depression or fenestra ; c, pos- 

 terior depression or fenestra ; 

 a., lobe divided off from anterior 

 part ; p.a., posterior or para- 

 chordal part of cranium ; x., 

 post-occipital plates. 



point strongly to its 



