THE SIPHONOPHORA. 



129 



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

 tacle, which occur at regular intervals. 



Pig. 26. — Athorybia rosacea.— The ends of the tentacular branches in various stages 

 of development. A, lateral branch, commencin<? as a bud from the tentacle. In 

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

 ty of the branch ; and, in ^, the sacculus is bei?inning to be marked off, and thread- 

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

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

 which is strai;j,ht, 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 hvdranth, in 

 which case they inclose the hydranth with its tentacle and a 

 group of gonophores (Calycop?ioridce)^ or, independently of 

 the hydranths, from the coenosarc (many Physophoridm). 



The hydrophyllia are transparent, and often present very 

 beautifuUj^ defined forms, so that they resemble pieces of cut 

 glass. Tliey 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 



