118 THE ANATOMY OF INVERTEBRATED ANIMALS. 



(Discophora) the aboral end of the hydranth is dilated into 

 a disk or umbrella^ which is susceptible of rhythmical con- 

 tractile movements, by which the body is propelled through 

 the water. Thus, notwithstanding its different mode of de- 

 velopment, it has a close resemblance to a medusoid. Ac- 

 cording to the existence or absence of these various append- 

 ages, and the manner in which they are disposed, the Hy- 

 drozoa are distinguishable into three groups — 1, the Sydro- 

 phora ; 2, the Discophora ; 3, the Siphonophora, 



1. The Hydeophoka are, in all cases but that of Hydra^ 

 fixed ramified hydrosomes, on which many hydranths and 

 gonophores are developed. The somatic cavity contained in 

 the hydrosoma always retains a free communication with the 

 gastric cavities of the hydranths. In other words, it is an 

 enterocoele. The tentacula are either scattered over the hy- 

 dranths (Cory7ie), or are arranged in one circle round the 

 mouth [Sertularia) ; or in two circles, one close to the mouth, 

 and one near the aboral end { Tubular ia). Very generally — 

 e. g., in all Sertularidcp, GamiJanidaridm and Tuhidaridce — 

 there is a hard, chitinous, cuticular skeleton (perisarc of All- 

 man), which frequently gives rise to hydrothec^e, into which 

 the hydranths can be retracted (Fig. 13, 2). 



The gonophores present every variety, from simple sac- 

 cular diverticula of the hydrosoma to free-swimming medu- 

 soids. The inner margin of the bell in these medusoids is 

 always produced into a velum, and otolithic sacs and eye- 

 spots are very generally disposed at regular intervals around 

 the circumference of the bell. The great majority of what 

 were formerly termed the naked-eyed MedusiB {Gymnoph- 

 thalmata) are merely the free-swimming gonophores of the 

 Hydrophora. Thus the medusoids known as Sarsiadce are 

 the free gonophores of the Corynidm ; the JSougainmllem 

 and Lizzice of the Eudendridoe ; many Oceanidm proceed 

 from Tubular id(B ; Thaumaiitidce and ^quoridcB from (7am- 

 panularidce. 



In some Hydrophora (e. g., Calycella) the margins of the 

 hydrotheca are prolonged into triangular processes, which 

 serve as an operculum. 



Certain PlumularidcB are provided with prominences of 

 the hydrosoma surrounded by a chitinous investment, which 

 is open at the extremity. The inclosed soft ectoderm usual- 

 ly contains many thread-cells, and has the power of throw- 

 ing out contractile pseudopodial processes. These have been 



