150 



ZOOLOGY. 



hang in clusters from the genital bands in the periviscenil 

 cavity. The mature eggs detach themselves from the ovary 

 to float freely in the perivisceral cavity, whence they pass into 

 the flaring, ciliated mouths of the segmental organs, and are 

 discharged by them into the water. These segmental organs 

 or oviducts are tubular, trumpet-shaped, as in the true 

 worms (Fig. 101). In Lingula, Discina, and Terebratulina, 

 there is but a single pair, in Rhynconella two pairs. The 

 external orifices of the oviducts form simple slits, while in 

 Terebratulina they project from the anterior Avails like 

 tubercles, as in the true worms (Morse) The spermaries 

 occur in the same situation in the perivisceral cavity as the 

 ovaries. As observed in Terebratulina, by Morse, in a few 

 hours after the eggs are discharged the embryos hatch and 

 become clothed with cilia. Kowalevsky observed in the egg 

 of Thecidium the total segmentation of the yolk (also ob- 

 served in Terebratulina by Morse), until a blastoderm is 

 formed around the central segmentation cavity, which con- 

 tains a few cells. The similar formation of the blastoderm 

 was seen in Argiope, but not the morula stage. After this 

 the ectoderm invaginates and a cavity is formed, opening 

 ^externally by a primitive mouth. The walls of this cavity 

 -now consist of an inner and outer layer (the endoderm and 

 ectoderm). This cavity eventually becomes the digestive 

 (Cavity of the mature animal. 



In Terebratulina Morse observed that the oval ciliated 

 germ became segmented, dividing into two and then three 



rings, with a tuft of 

 long cilia on the an- 

 terior end (Fig. 102, 

 A). In this stage the 

 larva is quite active, 

 s w i m m i n g rapidly 

 about in every direc- 

 tion. 



Soon after, the germ 

 looses its cilia and becomes attached at one end as in Fig. 

 102, B (c, cephalic segment ; th, thoracic segment; p, pe- 

 duncular or caudal segment). The thoracic ring now in- 



Fig. 105. Larval stages of Terebratulina. 

 After Morse. 



