GENERATION. 123 



this, numerous motile larvae are developed, which after a time 

 escape by rupture of the wall of the cyst, subsequently to 

 assume the form of the parent, most probably without meta- 

 morphosis. Occasionally from the sacculated stage an acineti- 

 form larva (so called from its general resemblance to the 

 genus Acineta) arises, from the interior mass of which Vorti- 

 cellce successively escape.* In Paramecium true generation 

 may occur following coition. Entire segmentation of the 

 ovum occurs, and the young almost directly assume the 

 form of the parent. In Trichodes an encapsulated stage ap- 

 parently intervenes between the first larval and mature con- 

 dition. 



RADIATA. Ccelenterata. Reproduction in Polypi by fis- 

 suration, gemmation, and true generation. In addition to 

 remarkable capacity to multiply by fissuration and gemma- 

 tion, the Hydra periodically produces young by true gener- 

 ation. It may be hermaphroditic or unisexual, but more 

 commonly the former. The development is without meta- 

 morphosis. In Actinia (sea-anemone), which is hermaphro- 

 ditic, the egg is impregnated in the water. After complete 

 segmentation it becomes ciliated. The embryo in time as- 

 sumes a horseshoe form, the outer boundary of which event- 

 ually constitutes the periphery of the animal, the inner 

 boundary, the common visceral cavity. 



In Acalephse are witnessed some of the most remarkable 

 methods of development known in the animal kingdom. 

 They are for the most part examples of alternate generation. 

 In Hydroida this process is particularly complicated. The 

 following terms will be employed in its description: 



Planula. The freed embryo. 



* Encapsulation resembles true generation in the following particulars : 

 1st. The 'nucleus' contained within the capsule is analogous to an ovum. 

 During encapsulation this undergoes segmentation, producing numbers of 

 distinct larval forms. 



2d. The presence of male and female organs within the capsule being 

 assumed, animals undergoing encapsulation are hermaphroditic, a conclu- 

 sion in harmony with facts observed in closely allied animals, the Spongida, 

 as in Tethva. 



