196 COMPARATIVE ANATOMY 



and the cavities themselves send back ventral prolongations 

 which extend into the atrial folds when the latter structures make 

 their appearance. Eventually the cavities in the atrial folds dis- 

 appear, and their walls are used up in the formation of the atrial 

 muscles. The anterior end of the archenteron sends forward 

 a pair of outgrowths, and is then constricted off from the rest 

 of the gut. The sac thus formed is divided into two vesicles 

 corresponding to the two anterior outgrowths, and so the first 

 pair of ccelom-sacs or " head-cavities " is established. The 

 head-cavities differ from all the other ccelomic sacs in the fact 

 that their walls never give rise to muscles. Their fate is 

 somewhat peculiar. The right cavity grows to a larger size 

 than the left, passes below the notochord, and forms the pre- 

 oral cavity or ccelom of the snout of the larva. It becomes 

 obliterated in the adult. The left cavity also is shifted below 

 the notochord, but its fate is very different, for it requires an 

 opening to the exterior on the right side, and becomes the 

 pre-oral sense pit, which has been described as a ciliated pit 

 on the inner side of the vestibule. 



The early developmental stages of Amphioxus are passed 

 through very rapidly. The blastula is completed about four 

 hours after the ova are extruded from the atriopore ; the gastrula 

 is completed about two hours later, the first mesoblastic 

 somites are formed four hours afterwards, and the embryo, 

 covered with a coat of cilia, escapes from the vitelline mem- 

 brane and swims in the water as a free larva. A few hours 

 later it has the form depicted in fig. 48, T, with an elongate 

 attenuated body, a pointed snout, a flattened caudal fin, a 

 swollen pharyngeal region, and the mouth and first gill-slit 

 have made their appearance. From this time onwards the 

 development progresses very slowly, and it is some months 

 before the larva is metamorphosed into the adult. 



The growth of the larval Amphioxus is characterised by an 

 extraordinary asymmetry in some of the more important 

 organs. Thus the mouth, when it is first formed, is an oval 

 aperture on the left side of the body. The first gill-slit is 

 formed as a ventral outgrowth of the floor of the gut, but as 

 soon as it has established communication with the exterior it 

 is shifted up on the right side. Other gill-slits, to the number 

 of fourteen or fifteen, are formed behind the first gill-slit, and 

 similarly these are formed on the ventral surface and are 



