LARVAL FORMS : THEIR NATURE, ORIGIN AND AFFINITIES. 393 
Paired excretory organs opening to the exterior and into 
the body cavity are often present. 
A B 
Fig. 10. — A. The Larva of a Holothuroid. B. The Larva of ati Asterias, 
m, mouth ; st. stomach ; a. anus ; l.c. primitive longitudinal ciliated 
band ; pr.c. praeoral ciliated band. 
This type of larva is found in the Potifera (fig. 3) (where it 
is preserved in the adult state), the Chsetopoda, theMollusca 
(fig. 4), the Gephyrea nuda (fig. 14), and the Polyzoa 
(fig.l5).i 
4. Tornaria. — This larva (fig. 16) is intermediate in 
most of its characters between the larvae of the Echinoder- 
mata (more especially the Bipinnaria) and the Trochosphere. 
It agrees with Echinoderm larvae in the possession of a 
longitudinal ciliated band (divided into a praeoral and a 
postoral ring), and in the derivation of the body-cavity and 
water-vascular vesicle from alimentary diverticula ; and it 
resembles the Trochosphere in the presence of sense organs 
Fig. 11. — A Larva of Strongylocentrus. (From Agassiz.) mouth; a. 
anus ; o. oesophagus ; d. stomach ; c. intestine ; v'. and v. ciliated 
ridges ; w. water- vascular tube ; r. calcareous rods. 
* For a discussion as to the structure of the Polyzoou larva, vide 
‘ A Treatise on Comparative Embryology,’ vol. i, p. 253. 
