THE SIPflONOPHORA, 



129 



ated in transverse reniform thickenings of the wall of the ten- 

 tacle, which occur at regular intervals. 



Fia. '2G.—Athor7/Ma rosacea.— The ends of the tentacular branches in various stages 

 of development. A, lateral branch, commencing,' as a bud from the tentacle. In 

 B, terminal papilliE, the rudiments of itie filaments, are developed at the extremi- 

 ty of tlie branch ; and, in C, the sacculus is beijinning to be marked off, and thread- 

 cells have appeared in its walls ; in />, the division into ii)volucrura and sacculus 

 is apparent; in B, the involucrum has invested the sacculus, the extremity of 

 which is straight, while the lateral processes have curled round it. 



Hydrophyllia are generally present, and, like the tentacu- 

 la, are developed either from the pedicle of a hydranth, in 

 which case tliey inclose the hydranth with its tentacle and a 

 group of gonophores (Calycophoridcji), or, independently of 

 the hydranths, from the ctrnosarc (many Ph]isophorld(je). 



The hydrophyllia are transparent, and often present very 

 beautifully defined forms, so that they resemble pieces of cut 

 glass. They are composed chiefly of the ectoderm (and meso- 

 derm), but contain a prolongation of the endoderm, with a 

 corresponding diverticulum of the somatic cavity. They are, 

 in fact, developed as cnecal processes of the endoderm and 

 ectoderm ; but the latter, with the mesodermal layer, rapidly 

 predominates. 



The gonophores of the Slpho7iophora present every varie- 

 ty, from a simple form, in which the medusoid remains in a 

 state of incomplete development, to free medusoids of the 

 Gynmophthalmatous type. As an example of the former 



