GENESIS OF MAN. 37 



The Gastrida is a common larval form of many lower animals, 

 such as Sponges, Polyps, Corals, Medusae, Worms, Mollusks, and 

 Radiates. It is also a larval form of the two most interesting of all 

 animals for the history of development : viz., the Ascidian and the 

 Amphioxus. Many Zoophytes and sponges are indeed nothing 

 more in their final state than an aggregation or society of Gastru- 

 lae. They therefore constitute a compound Gastraea. 



There is still another larval form belonging to this class which 

 possesses an almost equal interest with the Gastrula. This is the 

 Ascula. It belongs to the life histories of both Sponges and Me- 

 dusae, being developed out of the gastrula-form, and from it the 

 fundamental biogenetic law leads us back to the long extinct Pro- 

 tasciis, or primordial sack, which was the ancient progenitor of 

 the Zoophytes. It is fixed to the bottom of the sea, having its 

 open end directed upward. No longer needing the cilia em- 

 ployed by the Gastrula as organs of locomotion, these are conse- 

 quently wanting. Its body consists of a simple sack or stomach, 

 whose walls are formed by the two primary germinative layers in 

 all their primordial simplicity. 



The already famous Gastraea Theory of Haeckel is nothing 

 more than the simple application of his fundamental biogenetic 

 law to the Gastrula stage of development. By this law he is led 

 to the conclusion that at one period in the history of the globe, an 

 animal, having at maturity the form and organization of the Gas- 

 trula, and to which he gives the name of Gastraea, constituted the 

 highest form of organic development upon it, and that from this 

 primordial state of the two primary germinative layers, the process 

 of differentiation of organs proceeded, until the present complex 

 state of the animal kingdom has been reached, even as from the 

 embryonic gastrula-form the highest of living beings are now de- 

 veloped through this ontogenetic recapitulation. "The Gastraea," 

 says he, "must have lived at least during the Laurentian period, 

 and sported about in the sea by means of its ciliated exterior coat, 

 in the same manner as the free-moving Gastrulae now do." 



The great interest which attaches to the Gastraea Theory, as 

 already remarked, arises out of the immense importance of the 

 primary germ-lawyers as the basis of all future histological develop- 

 ment. That which carries it further out into the field of specula- 

 tion, however, and thus in one way adds still more to its interest, 



