On the Life-History and Development of the Genus Myzostoma. 555 



in an embryonic condition, and these, as will be seen, later lie on each 

 side of tbe alimentaiy canal in the hinder region of the larva. The anus 

 too, is now^ formed at the narrow or pearshaped end. To the ventral 

 side of the anus a papilla grows out, and on the dorsal side of this the 

 anus opens during larvai life. 



Middle period of Larvai history. 



The larva now enters upon a series of changes by which it is spe- 

 cially fitted for a course of free larvai existence. These changes con- 

 sist chiefly in the development of pro visionai setae, and in the restriction 

 of the ciliation to certain regions of the body. These things occur soon 

 after the formation of the mouth. The cilia are absorbed except in cer- 

 tain definite regions. On the »Scheitelfeld« or apex of the praeoral lobe 

 of the larva the cilia become diminished in numbers, but at the same 

 time they grow very much in length, and then form a crown of stiff cilia 

 on the Scheitelfeld (figs. 18, 19, 20). They are few in number, long 

 and fine and radiate somewhat from the centre, adding much to the 

 beauty of the larva. 



I bave never been able to distinguish a ring of praeoral cilia, and 

 in the absence of this I fear that the Myzostoma larva varies from Hat- 

 schek's ^ typical Annelidan larva. But seeing that in but few Anne- 

 lidan larvae this complete series of rings of cilia is developed , the ab- 

 sence of this particular ring cannot be taken as strongly militating 

 against the placing of Myzostoma among the Chaetopoda. No praeoral 

 circlet of cilia exists in this or any subsequent stage unless we regard 

 the crown of cilia in the praeoral lobe as the remains of one. This we 

 are hardly justified in doing, as will be pointed out later. The region 

 in front of the mouth, that is to say the praeoral lobe, in the larva is 

 very small and not well developed. Indeed the absence of any well 

 marked head region on the adult is reflected in part in the larva. But 

 a very well developed post-oral ring of cilia occurs (figs. 20, 21). The 

 cilia are long , mobile and pretty thickly set. They are especially nu- 

 merous and long directly behind the mouth on the ventral side. 



Further back we get a praeanal circlet of long cilia directly in 

 front of the anus, and a number of long stiffish cilia on the post-anal 

 papilla (figs. 21, 22). 



Simultaneously with the above alteration in the ciliation th^ larva 



1 Hatschek , Studien über die Entwicklungsgeschichte der Anneliden. Ar- 

 beiten a. d. Zool. Institut zu Wien. Bd. I. Heft III. 



