116 THE ANATOMY OF INVERTEBKATED ANIMALS. 



but it remains to be seen how far this rule is of general appli- 

 cation. 



Usually the region of the body in which the generative 

 organs are produced undergoes a special modification before 

 the reproductive elements make their appearance in it, giving 

 rise to a peculiar organ, the gonophore. In its simplest con- 

 dition the gonophore is a njere sac-like diverticulum, or out- 

 ward process of the body-wall. But, from this state, the 

 gonophore presents every degree of complication, until it ac- 

 quires the form of a bell-shaped body called from its resem- 

 blance to a Medusa or jelly-tish a medusoid.^ 



In its most complete form, the medusoid consists of a disk 

 having the form of a shallow or deep cup {iiectodyx)^ from the 

 centre of the concavity of which projects a sac termed the mn- 

 nubrium. The cavity of the sac is continued into that of 

 sundry symmetrically disposed canals, most commonly four in 

 number, w^iich radiate from the centre of the disk to its cir- 

 cumference, where they open into a circular marginal canal. 

 A membranous fold, the velum^ which contains muscular fibres 

 arranged concentrically to its free margin, is attached to 

 the inner circumference of the mouth of the bell, and pro- 

 jects, like a shelf, into its interior. Lithocysts are usually 

 developed on the margins of the bell, which may also give 

 rise to tentacles. The manubrium, opening at its free end, 

 may become functionally, as well as structurally, a hydranth, 

 and may serve to feed the medusoid when it is detached from 

 the hydrosomn^ or body of the hydrozoon. However com- 

 plex its structure may be, the medusoid commences as a sim- 

 ple bud-like outgrowth, which thickens at its free end ; the 

 central part of this thickening becomes the manubrium, 

 while its periphery, splitting away from the manubrium, is 

 converted into the disk (Fig. 13). A single prolongation of 

 the somatic cavity is continued into the manubrium, while 

 several, usually four, symmetrically arranged diverticula ex- 

 tend into the nectocalyx and become its radiating canals. 

 The distal ends of these subsequently throw out lateral 

 branches, which unite and give rise to the circular canal. 



The lithocysts are usually, but not always, free and promi- 



1 From the imperfection of our knowledge respecting the origin of many 

 of the medusiform Hydrozoa^ it is ditficult^to employ any terminology with 

 strict consistency. If "medusoid" is restricted to'wlia't are known to be 

 gonophores developed by gemmation, " medusa '' may be employed, in a gen- 

 eral sense, as the equivalent of the somewhat inconvenient vernacular term 

 "jelly-fish." 



