78 COMPARATIVE ANATOMY CHAP. 



stomach. The 4 septa which carry phaeelli or tufts of gastral fila- 

 ments are continued as gastric ridges or taenioles on the exumbrellar 

 gastric wall to the aboral pole of the body. In the higher Acraspeda, 

 the Discomedusce, the exumbrellar and subumbrellar walls of the 

 peripheral intestine (circumferential sinus) coalesce, so that here again 

 there arises a cathammal plate, in which variously -shaped radial 

 canals and radial pouches remain as survivals of the circumferential 

 sinus ; at such points the lamellae of the cathammal plate separate, 

 leaving between them spaces, the lumens of these canals. The deriva- 

 tion of the Discomedusce from a Scyphula form is further justified by 

 the fact that in many of them the Scyphula appears as an attached 

 early stage (Fig. 99, p. 130). 



The third form of the Cnidaria is the Ctenophora (Fig. 68). Its 

 body is ovate, with 2 dissimilar poles; its principal axis, which 

 connects the two poles, coincides with the long axis of the oval. At 

 one pole of the chief axis (the oral) lies the mouth. The opposite 

 pole here, as in other Cnidaria, is called the aboral pole. 



The oral aperture leads into a spacious cavity lying in the chief 

 axis, which has its rise, ontogenetically, through an invagination from 

 the exterior, and is lined, like the oesophageal tube of the Scyphozoa, 

 with ectoderm. We call this cavity the cesophageal cavity (" stomach " 

 of authors) (s). 



In form the cesophageal cavity is neither round nor radial, but 

 very much flattened ; in a transverse section its lumen appears like a 

 slit. In this we find the first departure from the radial body struc- 

 ture of the Medusa. A plane running in the direction of the flattened 

 cesophageal tube, and in which the chief axis lies, is called the median 

 plane (c-d). 



The oesophageal tube leads through another opening into a smaller 

 cavity lying above it and lined with endoderm the stomach (m). The 

 stomach is elongated at right angles to the chief axis and the median 

 plane, and thus, when the animal is viewed from either the oral or 

 the aboral pole, forms a cross with the oesophagus. A plane running 

 through the chief axis in the direction of the stomach stands at right 

 angles to the median plane, and is called the lateral plane (<?-/). 



The median and lateral planes thus cross each other at right 

 angles in the chief axis, just like the cross axes of the Medusa, each 

 axis consisting of two opposite perradii. While, however, in the 

 radially constructed Medusa the cross axes are quite similar, and the 

 planes which run through them in the chief axis divide the body into 

 four entirely similar quarters, the two cross axes in the Ctenophora are 

 not alike, and the lateral and median planes divide the body into four 

 quarters, of which only the two which are diametrically opposite are 

 similar. Either of the two planes by itself, however, cuts the body 

 into two similar halves. 



At the aboral pole of the Ctenophora, as opposed to all other Ccelen- ' 



