MORPHOLOGY. 5 



In fact, the growth of every hydrosoma is, absohitely or relatively, stationary towards one end 

 of its axis, while it takes place with rapidity towards the other. This growing extremity, 

 therefore, is, as it were, constantly moving away from the opposite end, and the open mouths 

 of all the polypites are turned more or less in its direction. Hence, I shall call this the distal 

 end, while the other, comparatively fixed, extremity may be regarded as proximal. The latter 

 is the basal or lower end in Hi/drida, Corynidce, and Scrtulariadce. It is the anterior end, in 

 the actively swimming JDipfit/es, the upper end, in Hippopodiiis and the Plnjsophorida. It is 

 the upper side of a Medusa, Ci/ancea, or Bhizostoma ; the lower, or attached, end of a Zucernaria, 

 and of a larval CyancEa or Rhizostorna. 



The distal extremity of the hydrosoma is always, so far as I am aware, either 

 csecal, or ends in a polypite, but is never modified into any other appendage. Tlie 

 proximal end is variously metamorphosed in the Hydrida, Corynidce, an^ Sertulariadce ; and 

 becomes a hydrorhiza, either expanding into a disc, or sending out many radicles, by which it 

 attaches itself to other bodies. 



In the Calycopliorida (PI. V, figs. 3 and 4), the proximal end of the coenosarc dilates a 

 little, and becomes ciliated internally, forming a small chamber, which gives off the ducts, by 

 whose intermediation the systems of canals, which embrace the cavities of the organs of loco- 

 motion, are brought into communication with the somatic cavity. At its upper end, this 

 chamber is a little constricted, and so passes, by a more or less narrowed channel, into a 

 various])^ shaped sac, whose walls are directly continuous with its own, and which will hence- 

 forward be termed the somafocyst. The endoderm of this sac is ciliated, and it is generally so 

 immensely vacuolated as almost to obliterate the internal cavity and give the organ the 

 appearance of a cellular mass. 



The somatocyst very commonly contains large, strongly refracting globules of an 

 apparently albuminous matter, of precisely the same character as those which may be observed 

 occasionally to pass through the pyloric valves of the polypites, into the somatic cavity ; 

 and I do not doubt that the globules result from the accidental accumulation of such products 

 of digestion. Not unfrequently an air-bubble may be seen in the somatocyst, whither it has 

 travelled, there can be but little question, by the same channel, being cither swallowed with 

 the prey or accidentally sucked in by a polypite, whose mouth has been raised above the 

 surface of the water. Such a chance bubble has of course no relation whatever with the air 

 contained within the float of a Physophorid; and it is somewhat surprising that any one 

 acquainted with both structures should have imagined the existence of even an analogy, still 

 less of a homology, between them. 



The float or pneiimaiophore characteristic and diagnostic of the Physophoridce is, 

 indeed, a most remarkable and well-defined structure (Pis. VI, VIII, &c.) 



In these ITydrozoa the proximal end of the coenosarc expands into a variously shaped 

 enlargement, whose walls consist of both the ectoderm and endoderm, and which encloses a 

 wide cavity in free communication with that of the coenosarc, and, like it, full of the nutritive 

 fluid. From the distal end or apex of this cavity depends a sac, v'ariously shaped, but always 

 with tough, strong, and elastic walls, composed of a substance which is stated to be similar 

 to chit'm in composition,' and more or less completely filled with air. 



^ At least in Velella. See Leuckart, Z. N. K., p. 114, 



