TRICHOPLAX, THE DICYEMIDS AND ORTHONECTID^E. 65 



g 



enibiyos (Fig. 31, V) similar to and developing directly into the 

 adult form. Another form of embryo is, however, produced 

 by older individuals, its formation being 

 accompanied by a peculiar behavior of the 

 germ-cells. The nucleus of each one first 

 divides into two unequal parts, the smaller 

 part separating as a parauucleus and under- 

 going no further development. The germ- 

 cell now segments, and an embryo (Fig. 

 31, e) consisting of a single large cell 

 partially surrounded by smaller cells re- 

 sults. The smaller cells are now thrown 

 off and separate somewhat from each other, 

 and the larger cell repeats the segmentation- 

 process, the smaller cells being again thrown 

 off; and this may happen three or four 

 times, the result being the production of 

 three or four concentric layers of small 

 cells surrounding a single larger one, all 

 lying in the central cell of the parent. The 

 large cell undergoes no further develop- 

 ment, but the smaller ones, except those 

 of the last generation, develop into " infu- 

 soriform " embryos of a peculiar and com- 

 plicated structure. The cells of the last 

 generation develop into " vermiform " em- FlG . si.-Dicyema ty- 

 bryos similar to those found in young pus (combined from 



Dicyemids several figures by 



The fate of the " infusoriform " embryos c = central cell, 

 has not been determined. Since they are e, = embryos. 



ciliated it seems not improbable that they (J = erm - c ells 



PI-IT ,- e ,1 . 7i = nucleus of central 



serve lor the dissemination ol the species ,, 



and its transference from one Cephalopod y= vermiform em- 

 host to another. It has, however, been bryo. 

 suggested that they may develop into males. 



THE ORTHONECTIDA. 



The Orthouectids are parasitic on Echiuoderms and 

 Nernertean worms and resemble in structure the Dicyemids, 



