154 THE ANATOMY OP INVERTEBRATED ANIMALS, 



hrachia^ Fig. 31), while in others the body is produced into 

 lobes [Callianh\i)^ or raaj even be ribbon-shaped {Cestum) ; 

 but, whatever their form, they present a distinct bilateral 

 symmetry, similar parts being disposed upon opposite sides 

 of a median plane, which is traversed by the axis of the 

 body. The mouth is situated at one end of this axis, which 

 ma}^ be termed the oral pole. At the opposite, or aboral 

 pole, there is no median aperture, but usually, if not inva- 

 riably, a pair of apertures a short distance apart. The faces 

 of the halves of the body present four longitudinal bands of 

 long and strong cilia, disposed in transverse rows, like so 

 many paddles; these constitute the chief organs of locomo- 

 tion. Each half is also often provided with a long retractile 

 tentacle ; and lobed processes of the body, or non-retractile 

 tentacula, may be developed on its oral face. The mouth 

 leads into a wide, but flattened, gastric sac, the aboral end of 

 which is perforated, and leads into a chamber termed the 

 infimdibiduin. From the aboral face of this, a canal which 

 bifurcates, or two canals, lead to the aboral apertures. On 

 opposite sides of the infundibulum a canal is given off toward 

 the middle of each half of the body, which sooner or later 

 divides into two, and these tw^o again subdivide, so that four 

 canals, which diverge and radiate toward the inner faces of 

 the rows of paddles, are eventually formed. Having reached 

 the surface, each radiating canal enters a longitudinal canal, 

 which underlies the row of paddles, and may give off branches, 

 or unite with the other longitudinal canals in a circular canal 

 at the aboral end of the body. In addition, two other canals, 

 wdiich run parallel w^ith each flat face of the gastric sac, open 

 into the infundibulum. And, when retractile tentacula are 

 present, their cavities also communicate with the same cham- 

 ber. 



The entire system of canals is in free communication with 

 the gastric cavity, and corresponds w^ith the enteroca^le of 

 an A-Ctinia. Indeed, an A^ctinia with only eight mesenter- 

 ies, and these exceedingly thick, whereby the intermesenteric 

 chambers would be reduced to canals; with two aboral pores 

 instead of the one pore, which exists in Ccreantlms ; and 

 with eight bands of cilia corresponding with the reduced 

 intermesenteric chambers, would have all the essential pecu- 

 liarities of a Ctenophoran. 



The question whether the Ctenophora possess a nervous 

 system or not is still under debate. Between the aboral aper- 

 tures there is a rounded cellular body, on which there is 



