MORPHOLOGY. 17 



into spermatozoa or ova. As an invariable rule, while the reproductive organ is in contact, 

 on the one side, with the nutritive somatic fluid, it is, on the other, directly exposed to 

 the water in which the animal lives. The genital apparatus is therefore always external, 

 and is never contained within the cavity of the body, a character which at once distinguishes 

 the Ilijdrozoa from the Actlnozoa. 



These organs are found in their simplest state in Hi/dra, in Lucernaria, and in many 

 Medusa, where they are distinguishable from other parts of the body only by their contents ; 

 but more usually they assume the form of pouches or sacs, termed t/onophores by Allman, 

 containing a diverticulum of the somatic cavity. Such is their form in llijdracthua, in 

 Coryne pusilla, and in some Plumularia. 



The reproductive organs of both sexes in Cordylophora, the female organs of Stephanomia, 

 Agalma, Athoryhia, Forskalia, Apolemia, and the male organs of Pliysalici, exhibit an 

 interesting series of transitions from the simplest to the most complex condition of the genital 

 apparatus in the Hydrozoa. For we find in all these cases that the reproductive organ is 

 no longer simple, but that its investment has acquired more or less the structure of a 

 nectocalyx, from whose walls those of the actual genital sac, the manubrium of Allman, tend 

 to become independent. To this investing part I apply the name of gonocahjx. 



In Tubularia hidivisa both the male and the female reproductive organs are included 

 within a gonocalyx, in which the four longitudinal canals are distinguishable while the organ 

 is yet young, though they appear to become obliterated with age. But the organs remain 

 attached, and their calyces, so far as I have observed, never e.xhibit contractions. A similar 

 condition of the reproductive organs has been observed in some CmiipanuJarKE. 



Hippopodius and Voyfia have, according to Kiilliker, long manubria (sperm-sacs and 

 ovisacs), surrounded at their bases by short and shallow hemispherical gonocalyces, which 

 have the four longitudinal canals united by a circular canal, characteristic of a complete 

 nectocalyx. 



The reproductive organs of both sexes of other Calycophorida, and the male organs, of 

 Athoryhia, Agalma, Apolemia^ Stephanomia, Halistemma, and Forslcalia, have a still more com- 

 plete, contractile gonocalyx, provided with its membranous valve, and the manubrium or 

 reproductive sac suspended from its roof either projects but very little beyond its aperture, or 

 is much shorter. In many of tiie Culycophoridce these organs become detached, and swim 

 about for a long time, propelled by the contractions of their calyces. 



From these conditions of the reproductive apparatus, the transition is easy to that presented 

 by those of both sexes in Laomedea yeniculata, many Corynidce, and Velella, and by the 

 female organs oi Physalia; in which the reproductive organ is detached before the develop- 

 ment of the generative products within the manubrial sac suspended from the roof of the 

 gonocalyx, and swims away as a medusiform zooid. Not unfrequently, this " Medusa has 

 to undergo great changes in size and form before attaining its sexually perfect state ; and it 

 is, consequently, necessary that it should feed and support itself for a considerable period. 

 To this end, the central sac or manubrium no longer remains a mere short receptacle, but 



^ " On the Structure of the Eeproductive Organs in certain Hydroid Polypes," ''Proceedings of the 

 Royal Society of Edinburgh,' Session 1857-8 j and " Additional Observations on the Morphology of the 

 Reproductive Organs in the Hydroid Polypes,-" Ibid. December 6th, 1858. 



3 



