PODAXONIA 



381 



The full embryonic history of Phoronis has not been satisfactorily 

 made out, although a preliminary account of the subject has been 

 given by Caldwell (1883 and 1885), and further work on the subject 

 has been done by Masterman (1898), Ikeda (1901), de Selys Long- 

 champs (1902), and Shearer (1906). 



The free-swimming larva of Phoronis is termed Actinotrocha, 

 and was regarded as an independent organism before its life-history 

 was known. Its remarkable metamorphosis into the adult form was 

 described by Metschnikoff (1871), while a minute description of the 

 structure of the adult larva was 

 given by Goodrich (1905). 



Masterman's paper awakened 

 widespread interest and created 

 a lively controversy. He en- 

 deavoured to show that Actino- 

 trocha, like the larva of Balano- 

 glossus (p. 575), possessed five 

 coelomic sacs, viz. a prae-oral 

 and two pairs of lateral sacs ; and 

 that these sacs were developed 

 as outgrowths from the gut, 

 and that consequently Phoronis 

 was allied to the Protochordata, 

 and in particular to Cepha- 

 lodiscus, which has ciliated 

 tentacles like those of the 

 Actinotrocha larva. He even 

 endeavoured to find the homo- 

 logue of the notochord in two 

 glandular pouches which project 

 forwards from the stomach of 

 Actinotrocha (<?/,- Fig. 311). 



The Actinotrocha larva 

 possesses a hood-shaped prae-oral 

 lobe covered with minute cilia and carrying a thickened apical plate. 

 Somewhat below the centre of its upper surface the prae-oral lobe 

 contains a cavity, called by Masterman the prae-oral coelom. 

 Behind the mouth there is an oblique ciliated band, in other words 

 a metatroch, which is drawn out into a series of hollow tentacles 

 (Fig. 309). The tentacles contain cavities which open into right and 

 left loop-shaped vessels, situated at the sides of the oesophagus, which 

 were compared by Masterman to the " collar coelomic cavities " of 

 Cephalodiscus. There are a pair of nephridial tubes which, according 

 to Masterman, open internally into the collar -cavities and are 

 compared by him to the collar pores of Cephalodiscus. Behind 

 these a pair of coelomic sacs flank the alimentary canal, which 

 correspond to the trunk coelomic cavities of Cephalodiscus. A 

 ciliated girdle or telotroch encircles the hinder end of the larva. 



rn.tr 



FIG. 308. Lateral view of the Actinotrocha 

 larva of Plwronis. (After Metschnikoff.) 



ap, apical plate ; b.v, blood-vessel ; m, mouth ; 

 m.tr, metatroch ; t.tr, telotroch. 



