LARVAL FORMS : THEIR NATURE, ORIGIN AND AFFINITIES. 387 
The one group contains the larva of the Coelenterata or 
Planula, the other group the larvse of all the other forms. 
C 
Fig. 2. — Three larval Stages of Eucope 'Polystyla. (After Kowalevsky. 
A. Blastopliere 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 ; hy. hypoblast ; al. gastric 
cavity. 
The Planula (fig. 2) is characterised by its extreme 
simplicity. It is a two- layered organism, with a form vary- 
ing from cylindrical to oval, and usually a radial symmetry. 
So long as it remains free it is not even 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 ancestral form of the 
Coelenterata. 
The larvee of almost all the other groups, although they 
may be subdivided into a series of very distinct types, yet 
agree in the possession of certain characters.^ There is a 
more or less dome-shaped dorsal surface and a flattened or 
concave ventral surface, containing the opening of the 
mouth, and usually extending 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 prceoral lobe. 
There is usually present at first a uniform covering of 
cilia ; but in the later larval stages there are almost always 
formed definite bands or rings of long cilia, by which loco- 
' The larva of the Brachiopoda does not possess most of the characters 
mentioned below. It is probably all the same, a highly dilferentiated larval 
form belonging to this group. 
