DEVELOPMENT. 439 
endoderm lies as an independent closed sac within the ectoderm, 
Meanwhile the embryo has become or is becoming free from the thir 
egg envelope, and begins to move about at the bottom in shallow 
water. It elongates and becomes more worm-like ; there is an anterior 
tuft and'a posterior ring of cilia; the primitive gut forms five coelomic 
pouches ; a mouth and an anus are perforated; there seem to be no 
fore-gut nor hind-gut invaginations. Two gill-slits appear; the regions 
of the body are defined at a very early stage. 
In the indirect development, there is a Tornaria larva, at first bell- 
shaped. A ventral mouth opens into the curved gut, which is furnished 
with a posterior terminal anus. A ‘‘dorsal pore” leads into a thin- 
walled sac which becomes the proboscis 
cavity of the adult. There are external 
bands of cilia, something like those of 
an Echinoderm larva, and also an apical 
sensory plate (like that of many Annelid 
trochospheres), with. two eye - spots. 
The Tornaria is a,pelagic form. During 
its period of free pelagic life it gradually 
loses its distinctive bands of cilia, be- 
comes diffusely ciliated, acquires a pro- 
boscis and two gill-slits, and thus ap- 
proaches the form already described. 
It is elongated in the post-oral region, 
and becomes a creeping form. The 
Tornaria must be regarded as the more 
primitive larval form; the temporary 
absence of mouth and anus in the other 
type is probably an adaptation acquired 
after the pelagic habit was lost. 
Johannes Miiller, ranked the Tornaria 
larva, whose adult form was not then 
known, beside the larvz of Echinoderms, 
and the resemblance has been recently 
emphasised by Willey. The ciliated 
bands of the Tornaria resemble those 
of Echinoderm larvee, but this is only 
a superficial characteristic. The an- 
terior pouch, which forms the cavity 
a 
Fic. 237.— Tornaria larva, 
from’ the side. — After 
Spengel. 
M., mouth; g.,- gut; «., anus; 
h., heart; g., pore entering 
proboscis cavity ; ¢.., anal ring 
of cilia: s.¢.7., secondary anal 
ring. he dark wavy line in- 
dicates the margin of the lobes 
of the larval body with their 
bands of cilia. Note also the 
apical spot with cilia and sense 
organ, 
of the proboscis and communicates 
with the exterior, has also been com- 
_ pared with the beginning of the water-vascular system in Echinoderms, 
and it is true that in both several independent ccelom pouches 
grow out from the primitive gut. The anterior body cavity in Balano- 
glossus communicates with the exterior by a pore, which becomes the 
proboscis-pore of the adult, and this has been compared with the 
water-pore, or outlet of the water-vascular system of Echinoderms, 
which similarly opens from an anterior enteroccel to the exterior. On 
the other hand, the presence of an apical plate—a structure almost 
invariably absent in Echinoderms—suggests an affinity with an Annelid 
trochosphere. ‘ 
