LARVAL FORMS. 



367 



Myriapoda, the Crustacean lame, and with the larval forms of 

 the Chordata. I shall leave these forms out of consideration. 



There are, again, some larval forms which may possibly turn 

 out hereafter to be of importance, but from which, in the present 

 state of our knowledge, we cannot draw any conclusions. The 

 infusoriform larva of the Dicyemidse, and the Cercaria of the 

 Trematodes, are such forms. 



Excluding these and certain other forms, we have finally left 

 for consideration the larvae of the Ccelenterata, the Turbellaria, 

 the Rotifera, the Nemertea, the Mollusca, the Polyzoa, the 

 Brachiopoda, the Chaetopoda, the Gephyrea, the Echinodermata, 

 and the Enteropneusta. 



The larvae of these forms can be divided into two groups. 

 The one group contains the larva of the Ccelenterata or Planula, 

 the other group the larvae of all the other forms. 



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

 plicity. It is a two-layered 

 organism, with a form varying 

 from cylindrical to oval, and 

 usually a radial symmetry. So 

 long 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 ancestral form of 

 the Ccelenterata. 



The larvae 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 common 

 characters 1 . There is a more or 

 less dome-shaped dorsal surface, 

 and a flattened or concave ventral surface, containing the open- 



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. 



Id 



ov 



FIG. 217. EMBRYO OF BRACHI- 

 ONUS URCEOLARIS, SHORTLY BEFORE 

 IT is HATCHED. (After Salensky.) 



m. mouth ; ms. masticatory appa- 

 ratus ; me. mesenteron ; an. anus ; 

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

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

 oesophageal ganglion. 



