546 



PHORONIDBA. 



the endoderm on each side, and partly as a pair of diverticula from 

 the anal pit. 



The fully-formed larva has a preoral lobe covered with cilia, a 

 ventral mouth, and a posterior anus, and, like so many larvae, has 

 a gelatinous, transparent appearance. The preoral lobe possesses an 

 apical thickening of ectoderm the larval ganglion and in some 

 species eye-spots. 



Behind the mouth there i a circlet of ciliated larval tentacles 

 (Fig. 439, Lt), and behind these again the rudiments of the adult 

 tentacles may be discerned. On the ventral surface behind the 

 tentacles an invagination of skin is formed which projects into the 

 body of the larva. There is a circlet of long cilia round the anus, 

 and the cilia on the margin of the preoral lobe are longer than the 

 rest, constituting a velar or preoral ring. It is this ring, combined 

 with the preoral lobe and apical plate, which has led naturalists to 



regard Actinotro- 

 clia as a modified 

 trochosphere larva. 

 But this type of 

 larva is found in 

 so many widely 

 divergent groups 

 (e.g. Mollusca, 

 Annelida, Echino- 

 dermata, Entero- 

 pneusta), that too 

 much attention 

 must not be paid 

 to it as a sign of 

 phyletic affinity. 

 In this connec- 

 tion it should be 

 noted that Ac- 

 tinotroclia differs 

 from the trocho- 

 sphere of Molluscs 

 and Annelids in 

 possessing a coelom 

 in the form of two 

 pairs of mesoblastic 

 sacs which not only 



FIG. 439. a, Metamorphosing Actinotrocha with ventral invagi- 

 nation evaginated at S (after Schneider), a, anus with circle of 

 cilia ; D intestine ; Lt larval tentacles. The preoral lobe is the 

 part of the larva above Lt (in the position of the drawing). It 

 overhangs the mouth, which is on the right hand side of it in 

 the drawing. 6, Young Phoronis (after Metschnikoff). D intes- 

 tine ; A anus ; T adult tentacles ; Vc circular blood vessel ; VI 

 longitudinal blood vessels. 



