CHAPTER XVIII 



THE LOWER VERTEBRATES 



THIS volume began with studies relating to Mammals and Birds, 

 which are the two highest classes of backboned animals; then 

 followed a short chapter on Reptiles, which occupy a median 

 position among vertebrates ; Amphibians and Fishes were then 

 discussed, and thus we are now brought near the base or the 

 beginning of the vertebrate series. But while it may be said 

 in a general way that Mammals, Birds, Reptiles, Amphibians, 

 and Fishes are the five classes of backboned animals or verte- 

 brates, it is no longer possible to rest satisfied with this state- 

 ment. There are still simpler vertebrates which cannot be 

 ranked among Fishes, if the word " fish " is to mean anything 

 precise. We therefore propose to conclude this section of the 

 volume with a short reference to the vertebrates lower than 

 Fishes, namely 



(a) Cyclostomes or round-mouths, e.g. lamprey and hag. 



(&) Lancelets, e.g Amphioxus. 



(c) Sea-squirts or tunicates, e.g. Ascidia. 



(a) CYCLOSTOMES OR ROUND-MOUTHS 



In some parts of the country schoolboys are familiar with what 

 they call " niners/' small slippery, eel-like creatures with a horse- 

 shoe shaped mouth, which are found in slowly flowing reaches 

 of streams. These " niners " are the young or immature stages 

 of sea lampreys Petromyzon marinus or of freshwater lampreys 

 (the lampern, P. fluviatilis and the sand-pride, P. planeri). If 

 some specimens can be captured uninjured they will be interesting 

 temporary additions to the aquarium. 



The sea lampreys are relatively large animals about a yard 



