26 Invertebrate Zoology. 



A CTENOPHORAN 

 (Mnemiopsis leidyi) . 



External Anatomy. The elongated gelatinous body 

 will be observed to have a broader, two-lobed oral pole and 

 a narrower aboral pole. From the latter eight series of 

 combs, ctenophoral rows, or swimming plates extend meri- 

 dionally towards the broader end, the two lobes of which 

 (terminal lobes) may be folded together and cover certain 

 structures. The month is a slit-like opening which extends 

 from lobe to lobe, its major axis thus determining the first 

 perradius, and the plane (sagittal or median plane) passing 

 through it and the axis of the body would divide the animal 

 into symmetrical lateral halves. 



A pair of small papilli-form organs will be noted, one on 

 either side of the mouth. These are the tentacular lobes, 

 and a plane passing through them and the axis of the body 

 at right angles to the sagittal plane, would divide the animal 

 into an anterior and a posterior half, the one an exact coun- 

 terpart of the other. Such a plane may then be called the 

 transverse or lateral plane. It lies in the second perradius. 



The true tentacles are considerably reduced. Each ap- 

 pears as a small opaque spot, lying in a small pocket, midway 

 between the lateral ctenophorial rows and a little aboral to 

 the tentacular lobe. 



The ctenophoral rows are of different lengths. The four 

 longer lie near the sagittal plane and extend over the ter- 



