ii. 3 AMPHIOXUS 25 



lancelet, which shows nearly all of these features in diagrammatic form. 

 Study of amphioxus will go a long way to show the basic plan on which 

 all later chordates are built, and, indeed, gives us a strong indication 

 of what the early chordates must have been like. 



Though it can swim freely through the water, amphioxus is essenti- 

 ally a burrowing animal, and many of its special features are connected 

 with this habitat. It lives in the sand, at small depths, and has been 

 found all round the oceans of the world. Evidently, in spite of its 

 simplicity, it is a successful type. It is found on British coasts and, 

 indeed, the first individual described was sent (preserved) from Corn- 

 wall to the German zoologist Pallas, who supposed it to be a slug and 

 called it Limax lanceolatus (1774). It was first figured and given the 

 name Amphioxus lanceolatus by Yarrell in 1836. However, the name 

 Branchiostoma had been given in 1834 by Costa and by the rules of 

 priority this is the official name of the genus. We may keep amphioxus 

 as a common name. Some eight species of Branchiostoma are recog- 

 nized, and in addition there is a group of six species referred to the 

 genus Asymmetron. These resemble Branchiostoma in general organi- 

 zation, but they have gonads only on the right side. 



The adult Branchiostoma lanceolatum is rather less than 2 in. long 

 and has the typical fish-like organization, whose main external 

 features are related to the methods of locomotion and feeding (Fig. 

 5). The body is elongated, and flattened from side to side. The skin 

 has no pigment, and the muscles can be easily seen as a series of 

 blocks, the myotomes, serving to bend the body into folds. As the 

 name implies, the body is pointed at both ends; there is no recogniz- 

 able head separated from the body. Indeed, there are no separate eyes, 

 nose, or ears, and no jaws, so that the fundamental plan of chordate 

 organization appears in almost its fullest simplicity from one end of 

 the body to the other. The front end is, however, marked by a series 

 of buccal cirri, which form a sieve around the opening of the oral hood 

 and are provided with receptor cells. 



Although the animal is provided with a large number of gill-slits 

 these do not appear externally, being covered by lateral folds of the 

 body, which enclose a ventral space, the atrium, opening posteriorly 

 by an atriopore. The outside edges of the atrium project as a pair of 

 metapleural folds, giving the body a triangular shape in transverse 

 section. The alimentary canal opens posteriorly by an anus, in front 

 of the hind end of the body, thus leaving a definite tail — a region of 

 the body not containing any part of the alimentary canal. 



The general arrangement of the organs can best be understood by 



