50 THE HYDROMEDUSAE 



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

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

 between the oral and aboral pole the 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 buttresses. 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 exact position 

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



APPENDIX TO HYDROMEDUSAE. No. II. 

 Graptolithidae. 



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. Dendroidea. These forms often exhibit a marked re- 

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

 have been often dimorphic ; in Dictyonema rarum each branch presents 

 a common canal, from which are given off pairs of dimorphic thecae open- 

 ing in opposite directions (? hydrotheca and gonotheca). 



ORDER 2. Graptoloidea. These forms possess also a tubular skeleton 

 with a common canal, and thecae of an apparently Sertularian type. The 

 stem is stiffened by a solid axis (virgula) which lies in a groove of the 

 perisarc. The theca of the primary zooid (sicula) does not increase con- 

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

 dimorphism, pear-shaped capsules (Dawsonia, Fig. 56 6 ) being often found 

 close to or attached to a Qraptoloid. The sicula when perfect exhibits two 

 regions a smaller, slighter, embryonic chamber, continuous with which is 

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

 ally provided with a spine. 



SUB-ORDER 1. Monoprionidae (Fig. 56 1 to3 ' 5 ). The thecae in this 

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

 either in the same direction as the mouths of the other thecae (Mono- 

 graptidae, Leptograptidae) or in the opposite direction (Dichograptidae, 

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

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

 communication with one another. 



SUB-ORDER 2. Diprionidae (Fig. 56 4>7 ). The thecae in this group are 

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

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

 Diprionidan, and later bifurcates into two Monoprionidan stems, and by 

 (6) Dimorphograptus, in which the stem is at first Monoprionidan, then 

 Diprionidan. The colony may exhibit two (Diplograptidae) or four 

 (Phyllograptidae) rows of thecae ; the virgula is centrally placed, and each 

 row of thecae generally has a separate common canal of communication 



