50 THE HYDROMEDUSAE 



It is of elongate form, with a mouth at the lower pole. In section it is 

 soniewliat square for the greater part of its length, but nearly midway 

 betwoeu the oral and aboral pole tlie body is constricted by a groove ; at 

 this point the tissues of the four angles of the square section are continued 

 across the constricting groove as flying butti-esses. In the groove and 

 between the buttresses spring four bilobed paddles or wings, each lobe 

 carrying an otocyst The ectoderm is ciliated and provided with nemato- 

 cysts. The coelenteron, otherwise simple, is continued through the 

 buttresses ; the endoderm of the paddles is solid. "While the nematocysts 

 and otocysts undoubtedly place this form with Hydrozoa, its e.\act position 

 lias yet to be determined. Nothing is known of its development. 



Ari'KNDix TO Hydromedusae. No. II. 



Graftoliihidae. 



The forms generally included in this class are known only in a fossil 

 state, and are divided into three orders, which possibly bear but little 

 genetic affinity to one another. 



Order 1. Peitdroiilfa. These forms often exhibit a marked re- 

 semblance to Sertidarian colonies (Dendrograptus). The zooids appear to 

 have been often dimorjihic ; in Dictyonema rarum each branch presents 

 a common canal, from wliich are given off pairs of dimoriihic thecae open- 

 ing in opposite directions (? hydrotheca and gontUheca). 



Order 2. Graptolouhn. These forms possess also a tubular skeleton 

 with a common canal, and thecae of an apparently Sertularian tyi>e. The 

 stem is stiflened by a solid axis (virgnla) which lies in a groove of the 

 l)erisarc. The theca of the primary zooid (siculti) does not increase con- 

 tinuously in length. In this group also there appears to have existed a 

 dimorphism, pear-shaped capsules (Dawsonia, Fig. 56*) being often found 

 close to or attached to a Giajitoloid. Tlie sicula when perfect exhibits two 

 regions — a smaller, sligliter, embryonic chamber, continuous witli which is 

 a stronger, larger, and darker chamber ; the mouth of the latter is gener- 

 ally provided with a si)ine. 



Sun-OaPKR 1. Motioprionithe (Fig. 56 ' *"'•'). The thecae in this 

 sub-order are arranged on one side of the axis only. The sicula may face 

 either in tlie s;ime direction as the mouths of the other thecae (Mono- 

 graptidae, Leptdgraptidae) or in the opposite direction (Didiograptidae, 

 Dicranograptidae). The second theca is budded from the sicula, the third 

 from the second, and so forth, a common canal placing the thecae in 

 connnunication with one another. 



Sub-Order 2. Diprionithf (Fig. 56*'). The thecae in this group are 

 arranged on two or four radii from the axis. These forms are linked with 

 the Monoprionidans by (n) Dicranograptus, the colony of which is at first 

 IHl>rionidan, and later bifurcates into two Monojirionidan stems, and by 

 (/)) Dimorphogra]itus, in which the stem is at first Monoprionidan, then 

 Diprionidan. Tlie colony may exhibit two (Diplngraptidae) or four 

 (Pliyllograptidae) rows of thecae ; the virgnla is centrally placed, and each 

 row of thecae genenilly has a separate common canal of communication 



