bihäng till k. sv. vet.-akad. handl. band 28. AFD. lY. n:o 7. 7 



exteriorl}- b}^ the original apertiire on the larval skin where 

 the ectodermic invagi nation took place. It was said before 

 that the five bladder-shaped folds have become flattened into 

 laminpe and liave finally joined at tbeir borders. On account 

 of the mode of formation of this membrane, it must evidently 

 have thin double walls limiting a third cavity situated between 

 the two former. Moreover, it can easily be understood that 

 this third cavity bas five interradial openings just at those 

 2)laces where the bladder-folds have protruded. It eonse- 

 quently commnnicates with the space between the ectodermic 

 invagination and the left posterior coelom. Through this 

 arrangement. it becomes easy not only for the wander-cells 

 but even for the coelom to enter into the cavity in question. 

 Considering that this double membrane with its cavity, contai- 

 ning wander-cells and parts of coelom, forms the first trace of 

 the buccal menbrane of the future Echinus, it may properly 

 be called »huccal membrane». This so much the more that 

 the mouth and lips originate from it, 



While these changes in the development have been going 

 on, the left posterior coelom has grown round the hydrocoele 

 and its wall has increased considerably in thickness where 

 it is in contact with the hydrocoele and the ectodermic inva- 

 gination. Even in an early developmental stage a set of 

 interradial folds of the coelom have commenced to make their 

 appearance. These increase rapidly, giving rise to several 

 sinus, the most important of which is the -»peripharyngeal 

 sinus», which surrounds the futnre •»AristotWs lantern». They 

 also send out prolongations which insinuate themselves between 

 the hydrocoele and the ectodermic disc. From these prolon- 

 gations evidently also give rise to the >:>perili(]emal system-». 

 Finally, the peripharyngeal sinus appears to separate itself 

 by means of constriction from the main-portion of the coelom. 

 Five other interradial protrusions of the left posterior coelom 

 run out and enter the cavity of the buccal membrane, thus 

 forming a kind of »jjerihuccal sinus». 



The development of the »dental system» is very difficult 

 to follow clearly and I am by no means sure of the correctness 

 of the conclusions I have hitherto reached. The »tooth» itself 

 and its two »hal f pyramids», one on each side of it, make their 

 appearance almost simultaneously in an early developmental 

 stage of the sea-urchin. A little låter on, the »epipJiyses» 



