458 



PHORONIS 



multiplication and growth of the epithelial cells which cover these 

 diverticula. When ripe the ova and spermatozoa drop off into 

 the body-cavity and make their way to the exterior through the 

 duct of the kidney. 



The ova are probably fertilised in the sea -water ; they 

 undergo the early stages of their development whilst entangled 



FIG. 231. Three stages 

 in the metamorphosis 

 of the Actinotrocha 

 into P/wronis. A, 

 Actinotrocha larva 

 with the iuvagination 

 (c), which will form 

 the trunk of the Pho- 

 ronis larva beginning 

 to appear. B, Stage 

 with the invagiiiation 

 partly extruded. C, 

 Stage when the extru- 

 sion is complete and 

 the alimentary canal 

 has passed into it. C 

 is after MetschnikofF. 

 , Mouth ; b, anus ; 

 c, invagination which 

 ultimately forms the 

 greater part of the 

 body of the adult. 



amongst the tentacles of the parent. The larval form to which 

 they give rise was known long before its connexion with the 

 adult was demonstrated by Kowalevsky. 1 It is known as the 

 Actinotrocha (Fig. 231, A), and according to Caldwell has the 

 following structure in Ph. kowalevsJcii. The mouth is anterior, 

 and the anus terminal and posterior ; the mouth is overhung by 

 an immense prae-oral lobe, which bears a special larval nerve 



1 Zapiski Acad. St. Peter sb. vol. xi. No. 1, 1867 (Russian). Abstract in Arch. 

 Naturg. Jahrg. xxxiii. 1867, Bd. ii. p. 235. 



