GENERATION AND DEVELOPMENT. 25 



The tentacula originate in crecal diverticula either of the wall of the polypite or of 

 some part of the coenosarc, and, in their simpler forms, merely elongate and acquire 

 thread-cells in their ectoderm, while their endoderm may or may not undergo that process of 

 vacuolation which, in some cases, obliterates the central canal. 



In all the Calycoplioridce and Phijsophoridce which I have examined, except Velella and 

 Porpita, the canal of the tentacle remains freely pervious, its endoderm being thin and 

 not vacuolated. When the tentacle is branched, its branches appear successively as close-set 

 buds on one side of the proximal end of the tentacle, the younger buds being always, as I have 

 stated above, developed on the proximal side of the older ones. The buds are, at first, simple, 

 short caeca, and such, with a slight change in form, they remain in Phi/salia; while in 

 Bhizoplii/sa they elongate and become filiform. 



The complex tentacular appendages of the Caljjcopliorida (PI. V), of Alhoryhia (PI. IX), 

 or Physojjliora (PI. VIII), arise in the same way, but, as the bud elongates, it becomes divided 

 into three portions : a proximal, slender division (the peduncle), with walls of the same thick- 

 ness all round ; a median, thickened, cylindrical sac, whose walls are much thicker on one 

 side than on the other ; and a distal, more slender, cylindrical portion. Shortly after this 

 has taken place, the rudiments of the large, oval, and small palisade-like, thread-cells appear 

 in the thick wall of the median division, which gradually acquires the characters of the sac- 

 culus. In the CalycoplioridcB as in Halistemma and StejjJianomia, the distal division elongates 

 very greatly, and becomes the single filament. In Athoryhia and those other Physophoridce 

 which liave a median lobe and two filaments, it divides at a very early period into three 

 lobes; the lateral ones elongate into the filaments, and acquire numerous thread-cells in their 

 thick walls, while the walls of the median process or lobe, which undergoes far less elonga- 

 tion, remain thin and free from thread-cells. 



The involucrum is formed as a process of the ectoderm of the distal end of the 

 peduncle. In Physophora, as will be more particularly described below, the distal end of 

 the peduncle itself undergoes a singular dilatation, and helps to form the envelope for 

 the sacculus. 



The rudiments of the hydrocysts are at first indistinguishable from those of the polypites ; 

 and, indeed, as I have already remarked, they are in their perfect condition like nothing 

 so much as polypites which, having reached a certain stage of development, have subse- 

 quently merely increased in size. 



A hydrotheca is simply the cuticular investment of its polypite, and, in the young bud, 

 is represented merely by the outer layer of the ectoderm. But, as growth proceeds, 

 this becomes more and more widely separated from the body of the polypite, assumes its 

 characteristic shape, and eventually opens at its distal end so as to allow of the protrusion 

 of the distal moiety of the polypite. This process of development may be very easily traced 

 in t\\e CampanularicE. 



Hydrophyllia, on tlie contrary, are always developed as buds precisely similar in 

 composition to those which give rise to polypites or tentacles, and are therefore composed of 

 both ectoderm and endoderm. As they grow they assume their characteristic form, the 

 ectoderm enlarging out of all proportion to the endoderm and its contained diverticulum of 

 the somatic cavity. Consequently, when they have attained their full growth, they appear 



4 



