JELL Y-FISHES. 



99 



has for a function the support of the axis in the -water. In Agahna, however it 

 is so sn.iall that its functional importance in this respect is very sliglit. The axis 

 of the Agahna is divided into two regions, one of whicli lies adjacent to the float 

 and is called the nectosteni, and the other, more distant from the same, the polyii- 

 steni. In larger specimens the length of tlie nectostem is about one-third that of 

 the polyjistem. The former bears a number of appendages of interesting chai-- 

 acter called the nectocalices. These bodies are situated in two rows or series, and 

 are glassy clear in their transjiareucy. Their union with the stem is of a very fragile 

 nature, and easily ruptured when the animal is raised out of the water. If we examine 

 a single nectocalyx we shall iind th<nt it resembles closely a medusa bell (hvdroid 

 gonopliore) in which the walls h.ave a more or less polygonal shajse. This form is tlie 

 result of a flattening of two opposite sides of the nectocalyx in order that it may ilt 

 closely in the series of which it is a member. Each nectocalyx has a cavity within, 

 « hich opens into the surrounding water through a circular orifice, partly closed by a 

 thin, washer-shaped body called the veil. The apex of the nectocalyx is situated 

 t)p|iosite the external opening, and marks the pohit of union of the bell and the necto- 

 stem. On either side of the apex, embracing the nectosteni, tlic bell walls arc con- 

 tiinied into gelatinous horns which closely interlock Avitli similar ]>niiecti(>iis i'l'om 

 nectocalices situated in the opposite series. 



The arrangement of the nectocalices on the nectostem is as follows : There are two 

 rows or series of these bodies placed diametrically opposite each other on the axis. 

 Each series is composed of a number of nectocalices jilaced one above the other, fitting 

 closely together by tlie flat faces on the outside of these bodies. The gelatinous horns 

 already mentioned interlock with corresponding bodies from the opjiosite series. By 

 the close approximation of adjacent bells on their flat faces, and the interlocking of 

 bells from opposite series, a certain rigidity is given to this jiortiou of the animal, 

 notwithstanding the delicate attachment to the stem. 



The disposition of the nectocalices causes all the bell openings in cacli scries to 

 point in the same direction, or almost at right angles to the length of the axis. The 

 action of the nectocalices is as follows : They are, as their name implies, structures 

 for a propulsion of the Agalnia from place to place through the water. When water 

 is taken into their bell cavities, by a violent contraction of the bell walls it is violently 

 forced out through the opening into that cavity against the surrounding water in 

 which the medusa is floating. The necessary result of this action is that the animal 

 is forced through the water in an opposite direction from that in which the resistance 

 takes place. By a nice adjustment of the different bells, acting in concert or independ- 

 enth', almost any motion in any direction can be imparted to the Agahna. .lust 

 below the float on the nectosteni there is a small cluster of minute bu<ls in which can 

 be found nectocalices of all sizes and in all conditions of growth. 



The attachment of the nectocalyx or swimming bell to the nectostem, not only 

 serves to move the animal from place to ]ilace, but also renders it possible for the 

 swinuning bell to receive its nourishment. Although the nectocalyx resembles very 

 closely a medusa, it is a medusa bell without a mouth or stomach. It is not cajjablc 

 of captiu'ing nourishment for itself, but is dependant upon others for that purjjosc. 

 Tlie nectocalyx has a system of tubes on its iinier bell walls which communicate with 

 the cavity of the nectostem by means of a sm.all vessel which lies in the peduncle by 

 which it is attached. Tln'ough this system of tubes the nutritive fluid is sujijilied to 

 the nectocalyx from a common receptacle, the cavity of the stem. In the largest 



