THE SIPHONOPHORA. 



129 



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

 tacle, which occur at regular intervals. 



Fio. 'Ifi.Athorytrla, 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 papillae, the rudiments of the filaments, are developed at the extremi- 

 ty of the'branch; ana, in C, the saceulus is beginning to be marked off, and thread- 

 cell? 1 have appeared in its walls ; in />, the division into involucrum and sacculus 

 is apparent: in h\ the involucrum has invested the saccnlus, the extremity of 

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



Hydrophyllia 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 Physophoridce). 



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 caecal processes of the endoderm and 

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

 predominates. 



The gonophores of the Stphonophora 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 



