

54 



HYDROZOA. 



YNIDJE 



In Cordylophora, the free swim- 



ming ciliated embryo, on emerging from 



(fig- 8, b) 



the 



_ _ _ . ,. „ J an 



elongated oval form, but very contractile, so that 



often it assumes a pyriform figure (Jig. 8, c and cl\ 

 Eventually, the embryo loses its cilia, and, fixing 

 itself, developes a hydrorhiza at one extremity 

 and a mouth at the other, thread-cells being at 

 the same time formed in the ectoderm {fig. 8, « 



Next, a series of about four tentacles 

 make their appearance ; these are soon succeeded 

 by others ; the somatic cavity becomes fully formed, 

 and the young Cordylophora, increasing in size, 

 is invested with a delicate cuticular layer. The 

 rudimentary ccenosarc, with its single polypite, 

 formed in this manner, soon commences to send 

 forth prolongations, and these, by gemmation 

 develop the polypites and other appendages of the 

 adult organism. 



A somewhat different series of changes occurs 

 in Tubularia {fig. 9 ). The embryo of this genus 



> 



makes 



as 



a discoid body, from the circumference of which 

 short thick processes, the rudiments of tentacula, 

 are produced (fig. 9, /). The disc then becomes 

 more gibbous at the side turned away from the 

 axis of the gonophore ; a mouth, leading into a 

 newly-formed digestive cavity, soon occupying the 

 centre of the opposite side. The mouth then 

 elevates itself on a conical prominence, around 

 which a second series of tentacles arise. In this 

 state the embryo issues from the gonophore {fig- 9> 

 d). Eemaining free for a short time, it finally 

 becomes fixed, and developes a coenosarc with its 

 cuticular layer (fig. n, e and a). 













