MORPHOLOGY. 15 



are confined to the latter. Where nectocalyces are present, the series of hydrophylHa always 

 ends on their distal side, these organs being never found intermixed with the nectocalyces, 

 or on their proximal side. The VeleUidce, P/it/saliache, Rhizoplnjsid(r>, and Fhysophoriada 

 have no hydrophyllia. 



6. Nectocalyces. 



Nothing is more peculiarly characteristic of those orders of the ITijdrozoa to which tlie 

 following pages are particularly devoted, than the structure of their locomotive organs, 

 which, whatever the modifications they undergo, may be reduced to a very simple type 

 — that of a cup lined by a muscular membrane, by whose contractions the water is expelled, 

 the animal being consequently urged, by its reaction, in the opposite direction. Nor 

 is the essential internal structure of this cup less uniform. Its summit contains a cavity, 

 which is connected with the somatic cavity on the one hand, and, on the other, sends 

 off at least four canals, whicli traverse the walls of the cup in a radiating manner, and 

 eventually open into a circular canal, which surrounds its mouth. These necfoccdi/cine canals 

 are lined by a continuation of the endoderm. The cavity of the cup, which, with its muscular 

 wall, may be termed the necfosac, does not communicate with this system of canals, 

 but is freely open externally, and, opposite the circular canal, its wall is always produced into 

 a contractile, valve-like membrane, which, when at rest, projects inwards all round the 

 aperture, and narrows it to a greater or less extent (Pis. I, Nl, VIII). 



The presence of the valvular membrane at once distinguishes a nectocalyx from an umbrella. 



These organs of propulsion^ are only known to exist among the Calycoplioridm, where 

 they are universal, and the Pliysophoridce, which only partially possess them. 



In the typical Calycophoridcn {Diphyes, Ahyla, Galeolaria), there are normally only 

 two nectocalyces, which are attached upon opposite sides of the proximal end of the 

 hydrosoma, but in Voylia they are numerous, and in Hippopodius, according to Leuckart, 

 amount to as many as twelve or more. In the FhjsopUoridtB they are often extremely 

 numerous, and (though all originally developed on one side of the coenosarc) are, in the 

 adult state, either arranged in two opposite series, or in several series, which are disposed in a 

 radiating manner around the proximal end of the ccEnosarc. 



In the Physophoridcs the inner faces of the nectocalyces are usually excavated 

 to enclose the ccenosarc, and those of opposite sides simply alternate with, and fit in between, 

 one another. Each nectocalyx is connected with the ccenosarc by a pedicle, traversed by 

 a tubular duct, which opens on the one hand into its apical cavity, and on the other into 

 the somatic canal, and is, in truth, nothing but the original base of the organ elongated. 



In all the Physopkoridw, and in most of the Calycophoridce, that portion of the coenosarc 

 which supports the nectocalyces lies in the same straight line with the rest; but in 

 Hip)popodius, Leuckart has pointed out that it is bent down, and, as it were, folded upon 

 the remainder of the coenosarc, whence the distal end of the series of nectocalyces may be 

 readily mistaken for the proximal, and vice versa. 



^ It is necessary to distinguish carefully between these simply locomotive organs and those similar 

 structures which are subordinated to reproduction, and contain a central gastric or genital sac. 



