154 THE ANATOMY OF INVERTEBRATED ANIMALS. 



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

 lobes (Callianira), or may 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 

 may 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 

 infundibulum. 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 two 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, 

 which run parallel with 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 with the enteroccele of 

 an Actinia. Indeed, an Actinia 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 Cereanthus ; 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 



