THE SIPHONOPHORA. 



129 



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

 tacle, which occur at regular intervals. 



Fig. ZQ.—Athorybia rosacea.— The ends of the tentacular branches in various stages 

 of development. -4, lateral branch, commencing as a bud from the tentacle. Iu 

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

 ty of the branch ; and, in (?, the sacculus is beginning to be marked off, and thread- 

 cells have appeared in its walls ; in Z>, the division into involucrum and sacculus 

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

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



Hvdrophyllia are generally present, and, like the tentacu- 

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

 which case they inclose the hydranth with its tentacle and a 

 group of gonophores (Calycophoridce), or, independently of 

 the hydranths, from the coenosarc (many Physophoridoz). 



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 csecal processes of the endoderm and 

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

 predominates. 



The gonophores of the Siphonophora present every varie- 

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

 state of incomplete development, to free medusoids of the 

 Gymnophthalmatous type. As an example of the former 



