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. 



(6) 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 



