CHAPTER XXII. 

 THE SKULLESS YERTEBRATES. 



As we have said, there is a great deal of difference be- 

 tween a backboned animal and a mollusc, a lobster, or an 

 insect; and yet there are Tertebrates without a backbone, 

 and without even a skull or brain, and which look so much 

 like worms or slugs as to have been mistaken for them. 

 Such an animal is the lancelet (Fig. 146). 



This animal (Amphioxus lanceolatus) lives in sand just 

 belcw low- water mark, ranging on our coast from the mouth 

 of Chesapeake Bay to Florida; it also occurs in other parts 

 of the world. 



From its worm-like form it was regarded as a worm by 

 some authors, and even as a slug or shelless mollusc. The 

 body is four or five centimetres (about 1^ inches) in length, 

 slender, compressed, pointed at each end. The muscular 

 segments are distinct to the naked eye. From the mouth 

 to the vent is a deep ventral furrow, and a slight fin ex- 

 tends along the back around the tail, ending in front of 

 the vent. 



The mouth is oval, surrounded with a circle of ciliated 

 tentacles. The moiith leads directly into a large broad 

 pharynx or ''branchial sac" (Fig. 146, g). 



The walls of this sac are perforated by long ciliated slits. 

 The water which enters the mouth passes out through 

 these slits, where it oxygenates the blood, and enters the 

 peribranchial cavity, thence passing out of the body through 

 the abdominal pore (Fig. 146,^). The pharynx leads to the 

 stomach (/), with which is connected the liver or caecum. 

 There, is a pulsatile vessel or tubular heart, beginning at 

 the free end of the liver, and extending along the under 



