LARVAL FORMS. 



303 



The Planula (fig. 216) is characterised by its extreme simplicity. 



C 



FIG. 216. THREE LARVAL STAGES OF EOCOFE POLYSTYLA. (After Kowalevsky.) 



A. Blastosphere stage with hypoblast spheres becoming budded into the central 

 cavity. 



B. Planula stage with solid hypoblast. 



C. Planula stage with a gastric cavity. 



ep. epiblast ; hi/, hypoblast; nJ. gastric cavity. 



It is a two-layered organism, with a 

 form varying from cylindrical to oval, 

 and usually a radial symmetry. So 



lon 



as 



it remains free it is not 



usually provided with a mouth, and 

 it is as yet uncertain whether or 

 no the absence of a mouth is to be 

 regarded as an ancestral character. 

 The Planula is very probably the an- 

 cestral form of the Cceleiiterata. 



The larvee of almost all the other 

 groups, although they may be sub- 

 divided into a series of very distinct 

 types, yet agree in the possession of 

 certain common characters 1 . There 

 is a more or less dome-shaped dorsal 

 surface, and a flattened or concave 

 ventral surface, containing the open- 

 ing of the mouth, and usually ex- 

 tending posteriorly to the opening of 

 the anus, when such is present. 



The dorsal dome is continued in 

 front of the mouth to form a large 



we- 



Id 



an 



ov 



FIG. 217. EMBRYO OF BKACHIONITS 

 URCEOLARIS, SHORTLY BEFORE IT IS 

 HATCHED. (After Salensky.) 



m. mouth; ms. masticatory appa- 

 ratus; me. ruesenteron ; an. anus; 

 Id. lateral gland; ov. ovary; t. tail 

 (foot) ; tr. trochal disc ; sg. supra- 

 oasophageal ganglion. 



prcBoral lobe. 



1 The larva of the Brachiopoda does not possess most of the characters mentioned 

 below. It is probably, all the same, a highly differentiated larval form belonging to this 

 group. 



