388 Prof. E. Forbes on the Moiyhology of the 



parts of zoophytes on their axis (exactlj' as the parts of plants), 

 supposing the shortening of the axis of a branch, the abortion of 

 the greater number of its tubular cells and their conversion into 

 spines, we can without much difficulty explain the essential na- 

 ture of this form of vesicle. 



3rd. Such wrinkled, ovate, coronated capsules as we see in 

 Sertularia rosacea and Plumularia pinnuta. These may be re- 

 garded as branches reduced to Avhorls of abortive cells, of which 

 the rugpe are the only traces, whilst the whorl of terminal cells 

 only exhibits a trace of its original nature and forms the corona. 

 That many cells enter into the composition of these highly me- 

 taraorphic vesicles is borne out by the fact that the vesicle of 

 Se7'fula?-ia rosacea is provided with an internal framework, con- 

 sisting of a slender axis and radiating processes, which is well 

 seen after the expulsion of the ova. In an exotic species the 

 composite natm-e of this form is further borne out by the fact of 

 its vesicles dehiscing longitudinally on one side. 



4th. Oblong, often triangular or flask-shaped, sometimes com- 

 pressed vesicles^ of which there are numerous examples among 

 our native species of Sertularia, as S. jwli/zo/iias, S. ahietina, S. 

 operculata, S. argentea, and the genera Thuiaria and Antennu- 

 laria. In these it would appear that all the cells had been me- 

 tamorphosed in the highest degree, and that the midrib is sup- 

 pressed for the formation of the ovarian cavity. In such species 

 of Sertularia as have alternate cells, the termination of this form 

 of vesicle is usually oblique ; in those which have opposite cells 

 it is straight. In Ant ennui ai-ia, where the cells are imilateral, the 

 beak of the vesicle is turned to one side. These facts afford strong 

 argviments in favom' of the Aiew I have taken of its essential 

 nature, further supported by the aiTangement of the vesicles in 

 relation to the branches on a species from the Cape (which also I 

 owe to INIr. Bowerbank), where the vesicles correspond to and 

 represent the branches. See Plate X. fig. 10. 



5th. The curious retort-shaped vesicles of Thoa Beanii and 

 Thoa halecina, which appear to be formed out of a branch re- 

 duced to a single joint and a single cell, which merged into each 

 other, form the ovarian capsule. 



6th. The apparently simple vesicles of Campanularia and Lao- 

 medea, which may possibly be single cells chlated. 



With the exception of the last-mentioned form of vesicle, of the 

 true nature of which I have not yet satisfied myself, the varieties 

 of form of these bodies, then, are all explainable on the theory 

 that they are metamorphosed branches, either branches of the 

 first order (primaiy axes), or of the second or third (secondary and 

 tertiary axes). They are severally explainable on the supposition 

 of union of pai'ts, or of suppression of some of the elements of 

 a branch, as of some of the cells, or of the central rib or axis, or 



