AND DEVELOPMENT OF MYBIOTHELA. 
575 
Fig. 14. Part of a blastostyle with gonophores, plasmodia, and claspers. 
a. Blastostyle ; b, b. Claspers ; c, c. Young gonophores ; d. A mature gono- 
phore, with the plasmodium escaping through its summit ; e. Walls of 
gonophore retracted and everted after the liberation of the plasmodium ; 
f Plasmodium liberated from the cavity of the gonophore, but still held in 
in its place by the spadix, and already seized by a clasper. The plasmodia 
( d andjf) present a lobed condition at the part turned towards the blastostyle, 
owing to the coalescence of their constituent plasma masses being here still 
incomplete ; g. A plasmodium entirely withdrawn by a clasper from its 
original position on the summit of the gonophore peduncle. 
Fig. 15. A male gonophore filled with the generating vesicles of the spermatozoa. 
Fig. 16. Mature free spermatozoa. 
Fig. 17. Structure of the plasmodium shortly after its seizure by the claspers. 
PLATE 58. 
Fig. 1. Planula. 
a. Ectoderm; b. Endoderm ; a. Cavity of planula ; d. External structure- 
less capsule. 
Fig. 2. Embryo after the walls of the planula had become invaginated to form the 
transitory arms. 
a. Body of the embryo ; b , A Orifices of involution ; c. External struc- 
tureless capsule. 
Fig. 3. Section through the centre of the embryo represented in fig. 2. 
a. Body of the embryo ; b, b. Arms formed by involution of the walls of 
the embryo ; c. External structureless capsule. 
Fig. 4. Embryo after the arms have become external by evagination. 
b, b. The evaginated arms. 
Fig. 5. Embryo after it has begun to elongate itself and acquire an oval form. 
a. Commencement of permanent tentacles ; b,b,b. Transitory arms. 
Fig. 6. Embryo after its escape from its capsule when it enters on its free life in the 
surrounding water. 
a. Distal extremity ; b. Proximal extremity ; c, c. Permanent tentacles ; 
d, d , d. Long transitory arms fully developed. 
Fig. 7. Embryo when it has begun to fix itself. 
a. Distal extremity with mouth ; b. Proximal extremity with disk of 
adhesion ; d, d, d. Transitory arms in process of disappearance. 
Fig. 8. The embryo has definitely fixed itself, and the transitory arms have entirely 
disappeared. 
