UNITIES OF LIFE 



cormophyta. Of the four stems of the coelenteria (which 

 have only a ventral opening and no gut-cavity) the 

 gastrasads remain at the gastrula stage, and the sponges 

 are formed by multiplication of the same stems of 

 gastraeads. On the other hand, the cnidaria develop 

 into higher radial (star-shaped) persons, and the platodes 

 into lower bilateral persons. From the latter are derived 

 the worms {vcrnialia), the common stem-groups of the 

 five higher animal stems, the unarticulated moUusks, 

 echinoderms, and tunicates, and the limb-forming artic- 

 ulates and vertebrates. 



A large part of the physiological advantages and mor- 

 phological perfection which the higher histona have, as 

 contrasted with the lower, may be traced to the circum- 

 stance that the tissue-forming organism articulates — that 

 is to say, divides on its long axis into several sections. 

 With this multiplication of groups of organs there goes, 

 as a rule, a more or less extensive division of work among 

 them, a leading factor of higher development. In this 

 point also we see the biogenetic parallelism between the 

 two great groups of the tissue-plants and tissue-animals. 



In the kingdom of the tissue-plants the articulated 

 cormophyta rise high above the unarticulated thal- 

 lophyta. While the articulation of the stem of the 

 former proceeds and leaves are developed at the knots 

 {nodi) between each two sections of the stalk, far greater 

 play is offered to polymorphic differentiation than in the 

 thallophyta, which are generally without this meta- 

 merism. The formation of the bloom in the flowering 

 plants or phanerogams consists in a sexual division of 

 labor among the thickly gathered leaves in a short 

 section of a stem. 



To the two groups of unarticulated and articulated 

 sprouts in the kingdom of the tissue-plants correspond, 

 in many respects, the two sections of the tissue-animals, 

 the unarticulated and the articulate. The two stems of 



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