ASTEEIAS PALLIDA. 



249 



undero-0 an additional thickenino:, and in some sections I have observed 

 indications of the intimate connection of their ciliary band with the 

 nervous system of the star. As a necessary consequence of this thicken- 

 ing of the ectoderm, which takes place all over the body, the star 

 which has just been formed is considerably smaller than the larva, as 

 may be seen at a glance on Tl. XIX. 



II. Gut. 



It was thought by Agassiz ['77] that the larval mouth and anus 

 persist. My observations, however, have led to a different result : 

 both the larval mouth and anus atrophy. In the following account 

 I shall therefore, adopting with trifling modifications, the termi- 

 nology of Roule [Embryologie comparée], speak of the jn-otostome and 

 meiastome and the j)rotoproct and metaproct. 



Frotostomc. — The protostome atrophies by a constriction of the 

 œsophagus and subsequent evagination of the portion which is con- 

 nected with the ectoderm ; but the exact stage and the precise point 

 at which this takes i)Iace seem to vary somewhat. As to the second 

 point it would be natural to expect that just so much of the œsophagus 

 would be evaginated as was invaginated during the formation of 

 the protostome, thus assigning the ectoderm and endoderm to then- 

 proper spheres in the adult body ; and I believe this is exactly what 

 takes place in nature. But at the same time we must admit that 

 there are variations from this rule, as will be seen from the de- 

 scription which follows. 



In stage E the oesophagus presents numerous irregular constric- 

 tions (Fig. 10, PL XIX), and its final division is effected at one of these 

 constrictions lying near the mouth, some time between stages F und 

 G. In one series of sagittal sections of a specimen but slightly 

 more advanced than stage F, however, the division was effected at the 



