628 



embryology: lancelet 



continuous perivisceral coelom. The cavities of the dorsal parts of 

 the somites remain separate and are known as myocoeles. Their 

 inner walls, against the notochord, become greatly thickened, each 

 to form a structure, known as a muscle plate, which gives rise to a 

 myomere in the adult, the walls between myocoeles giving rise to 

 connective-tissue septa between the myomeres, and the outer 

 walls of the myocoeles to the dermis. From the inner wall of each 



SAT U 



Fig. 486. — The development of the atrial chamber in By anchio stoma. — After 



Lankester and Willey. 



In I. the metapleural folds are seen sending a slight projection inwards. In II. the two projections have 

 united and enclose a small space {AT), which is the rudiment of the atrial chamber. In III. this space 

 is enlarging at the expense of the coelom. A comparison of this figvure with the cross-section of the 

 adult (Fig. 226) will show the relation of coelom and atrial chamber. 



FR., Ccelomic space within dorsal fin ; AL., gut ; S., coelomic space of metapleural fold ; MP., metapleural 

 fold ; SAT., projection which forms floor of atrial chamber AO., aorta B.C., coelom ; SIV., sub- 

 intestinal vein ; A'., nerve cord ; NCR., notochord ; SH., sheath of notochord ; MY., muscle plate ; 

 C, cavity of sclerotome ; AT., atrial chamber. The dotted hne indicates the mesodermic wall of the 

 coelom. 



epimere, below the muscle plate, an outgrowth burrows its way 

 between the muscle plate and the notochord and forms from its 

 wall the connective-tissue sheath of the notochord and nerve 

 cord. This outgrowth is known as the sclerotome, the main part, 

 which contains the muscle plate, being known as the myotome. 

 Lastly, in the pharyngeal region the myotome grows down in 

 the body- wall, between the splanchnocoele and the ectoderm, 

 and at its lower end forms an outgrowth, the gonotome, which 

 forms a gonad, 



HABITS OF THE LARVA 



During the external and internal changes which we have 

 traced, the larval amphioxus swims freely in the sea, usually 

 at a depth of a few fathoms from the surface. As its metamor- 

 phosis reaches completion, it sinks to the bottom and takes up 

 the burrowing habits of the adult lancelet (see p. 299). 



