THE HYDROMEDUSAE 5 



The coenosarc generally consists of a branching vertical stem (the 

 hydrocaulus), springing from a branching horizontal stolon (the hydro- 

 rhiza), by which attachment is effected to some foreign body. A trans- 

 verse section of either hydrocaulus or hydrorhiza typically presents 

 the same ectoderm, mesogloea, and endoderm lining coelenteron, as 

 are exhibited by a section of a Hydra or of its tentacle (Fig. 4, C). 

 Theoretically, in the Anthomedusae an axial stem or branch is only 

 the much elongated body of the terminal hydroid of that stem or 

 branch ; but as in practice it is often difficult to allot the parts 

 correctly, the tubular stems and branches are treated as coenosarc 

 or tissues common to the colony. 



The coenosarc is generally invested by a horny coat, the perisarc, 

 formed as a secretion by the ectoderm cells ; this in some cases 

 expands into a hydrotheca (Fig. 17) at the base of each hydroid, in 

 others (Fig. 16) it ceases abruptly at that point. 



Hydroids are formed either as buds from other hydroids, or as 

 buds from the coenosarc, or directly from a fertilised ovum ; they 

 are generally fixed, sterile, and nutritive. 



The MEDUSOID (medusiform person, gonozooid, gonophore) 

 exhibits all the parts of a hydroid, but in slightly altered relations. 

 It is generally bell-shaped (Figs. 5, 25, 33), the clapper of the bell 

 being formed by a projection (the manubrium), at the end of which is 

 the mouth. The bell itself is often termed the umbrella ; its oral or 

 concave face is styled the subumbral, and its aboral or convex face 

 the exumbral surface. From the lip of the bell or umbrella a shelf 

 (the velum) projects inwards, and the tentacles hang downwards. 

 The mouth opens through the manubrium into an expanded gastric 

 cavity ; from this four perradial canals lead to the lip of the bell and 

 there open into a circular canal which runs round its circumference. 



Although the relation of this organism to the hydroid is not 

 obvious at first sight, a comparison of Figs. 4 and 5 will make 

 it clear. The elongated hypostome of the hydroid corresponds to 

 the manubrium of the medusoid ; the tubular body of the hydroid, 

 if expanded radially outwards in every direction, would represent 

 the bell-shaped body of the medusoid ; the tentacles would be 

 carried outwards by this expansion, but would remain as a circlet 

 round the hypostome (manubrium). 



While the outward form of the medusoid is thus referable to 

 that of the hydroid, the coelenteron of the former is not of the 

 simple nature which is presented by that of Hydra ; the endo- 

 derm is no longer uniformly the lining of the coelenteron, but 

 forms a solid cup-shaped plate (the gastral or endoderm lamella), lying 

 in the wall of the umbrella between the gastric cavity and the circular 

 canal, except along certain lines which have been already cited as 

 the radial canals (Figs. 5, 6). The coelenteron thus consists 

 of the following regions, manubrial cavity, gastric cavity, radial 



