XIX] CYCLOSTOMATA 443 



The excretory function of the kidney tubules on this view would 

 not be their original one; it would be a secondary function which 

 had gradually supplemented the primary one. It must be remem- 

 bered that the function of the whole coelomic wall is excretory, and 

 any convenient opening which allows its contents to escape may 

 develop into an excretory tube. This change of function has almost 

 certainly occurred in Polychaeta, Annelida and in Mollusca, as well 

 as in Craniata. Since the archinephric duct opens into the terminal 

 portion of the alimentary canal behind, the faeces (as the indigest- 

 ible remnants of the food are termed), the true excreta and the 

 genital cells are all expelled through the same opening, which on 

 this account has received the name cloaca which the Romans 

 bestowed on their common sewer. 



DIVISION I. CYCLOSTOMATA. 



The Craniata are divided into two main groups, namely, the 

 Cyclostomata and the Gnathostomata. The former division, 

 distinguished by the absence of true visceral arches and of jaws, 

 includes at the present day only a few, probably degenerate, worm- 

 like animals, with short tails like Amphioxus, and with naked 

 skins. The name Cyclostomata means Round-mouthed (Gk. KVK\O?, 

 a circle ; ord/xa, mouth), and alludes to the circumstance that the 

 edges of the mouth are stiffened by a ring-shaped 

 cartilage, the annular cartilage, so that the mouth 

 cannot be closed (2, Fig. 220). There is a piston-shaped tongue 

 supported by a lingual cartilage, and the whole is protruded by 

 a muscle attached to the annular cartilage in the lips (Fig. 219). 

 Both the tip of the tongue and the walls of the stomodaeum are 

 beset with horny teeth, developed from the agglutinated cells of 

 the skin. The expansion of the stomodaeum causes the mouth to 

 act like a sucker, and the whole animal is thus enabled to adhere 

 to some foreign body, such as a stone, or to some victim, usually 

 a fish, in which case the rasp-like tongue works a hole in the flesh 

 of the prey. The stomodaeum is greatly elongated and is supported 

 in its roof by several broad cartilages, the so-called labial carti- 

 lages ; in consequence the eyes and gill-slits appear to be pushed 

 very far back. 



The condition of the sense-organs is one of the most marked 

 characteristics of the Cyclostomata. The nose is represented by a 

 single sac placed far back in consequence of the elongation of the 



