150 ZOOLOGY. 
hang in clusters from the genital bands in the perivisceral 
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 walls 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, 
swimming rapidly 
about in every direc- 
Fig. 102.—Larval stages of Terebratulina.— tion. 
eee Soon after, the germ 
looses its cilia and becomes attached at one end as in Fig. 
102, B (c, cephalic segment ; ¢h, thoracic segment; p, pe- 
duncular or caudal segment). The thoracic ring now in- 
