276 COELENTERATA HYDROZOA 



CHAP. 



In some of the more complicated colonies; however, the stem 

 is composed of several tubes, which may or may not be surrounded 

 by a common sheath of ectoderm and perisarc, as they are in 

 Ceratella among the Gymnoblastea. Such stems are said to be 

 " polysiphonic " or " fascicled." The polysiphonic stem may arise 

 in more than one way, and in some cases it is not quite clear in 

 what manner it has arisen. 1 



In many colonies the zooids are only borne by the terminal 

 monosiphonic branches, which receive the special name " hydro- 

 cladia." The gonophores of the Calyptoblastea are usually borne 

 by rudimentary zooids, devoid of mouth and tentacles (the 

 " blastostyles "), protected by a specially dilated cup of perisarc 

 known as the " gonotheca " or " gonangium." The shape and size 

 of the gonothecae vary a good deal in the order. They may be 

 simply oval in shape, or globular (Schizotricha dichotoma), or 

 greatly elongated, with the distal ends produced into slender 

 necks {Plumularia setacea). They are spinulose in P. echinulata, 

 and annulated in P. halecioides, Clytia, etc. 



In some genera there are special modifications of the branches 

 and hydrocladia, for the protection of the gonothecae. The name 

 " Phylactocarp " is used to designate structures that are obviously 

 intended to serve this purpose. The phylactocarp of the genera 

 Aglaophenia and Thecoearpus is the largest and most remarkable 

 of this group of structures, and has received the special name 

 " corbula." The corbula consists of an axial stem or rachis, and 

 of a number of corbula-leaves arising alternately from the rachis, 

 bending upwards and then inwards to meet those of the other 

 side above, the whole forming a pod -shaped receptacle. The 

 gonangia are borne at the base of each of the corbula-leaves. 

 There is some difference of opinion as to the homologies of the 

 parts of the corbula, but the rachis seems to be that of a 

 modified hydrocladium, as it usually bears at its base one or more 

 hydrothecae of the normal type. The corbula-leaves are usually 

 described as modified nematophores (vide infra), but according to 

 Nutting 2 there is no more reason to regard them as modified 

 nematophores than as modified hydrothecae, and he regards them 

 as " simply the modification of a structure originally intended to 



1 For a discussion of the origin of the polysiphonic stem in Calyptoblastea see 

 Nutting, "American Hydroids," Smithsonian Institution Special Bulletin, pt. i. 

 1900, p. 4. " 2 Loc. cit. p. 33. 



