CTENOPHORA. 717 



anterior or dorsal, and a posterior or ventral half. Each half consists of 

 two quadrants l . Two axes, a stomachal or lateral and a funnel or dorso- 

 ventral axis, pass respectively through the stomachal and funnel planes. A 

 horizontal plane at right angles to the planes of the principal axis may 

 be termed the equatorial or dorso-ventral plane. The two tentacles lie one 

 at each end of the funnel plane. The eight meridional vessels and cteno- 

 phoral rows are distributed four in each lateral half, or two to each quadrant 

 of the body. The row of plates or the vessel in each quadrant which 

 adjoins the funnel plane or tentacle, is termed ' subtentacular ' ; that which 

 adjoins the stomachal plane ' subventral.' 



The typical shape of a Ctenophore as seen in a Pleitrobrachia is ovate, 

 the long axis of the oval coinciding with the principal axis. The mouth 

 leads into a flattened ectodermic stomach, the plane of the flat sides coin- 

 ciding with the stomachal plane. Towards the aboral aspect, the stomach 

 leads in turn into the endodermic funnel, the longest diameter of which 

 coincides with the funnel plane. The funnel gjves origin to two 'per- 

 radial ' vessels, which run more or less horizontally outwards in the funnel 

 plane in opposite directions. At their outer ends these vessels bifur- 

 cate. And the four vessels thus formed belong one to each of the four 

 quadrants and are known as ' interradial ' vessels from the direction they 

 take. A vessel to each tentacle springs from the angle or fork between each 

 pair of interradials. The four interradial vessels bifurcate in turn giving 

 origin to adradial vessels, eight in number, two to each quadrant. The 

 adradials open one into each of the eight meridional or ctenophoral vessels 

 which end blindly at their oral and aboral ends. The eight rows of cteno- 

 phoral plates coincide with the eight meridional vessels. A vessel originates 

 from each of the perradial vessels close to its origin, runs in an oral direc- 

 tion close to the flat side of the stomach, and ends blindly. These two 

 vessels are the ' paragastric canals.' 



At the centre of the aboral pole is a pyramidal depression surmounted 

 by a hyaline bell. The depression is elliptical or roughly hexagonal in 

 outline, the longer axis of the ellipse or hexagon coinciding with the 

 stomachal plane. The space inclosed is the central nervous system or sen- 

 sory area. The ectoderm cells of this area are ciliated. Certain of them dis- 

 posed in a circular row are provided with very long cilia which are bent like 

 the figure 2, and are fused in groups to form four triangular cilia-plates. 

 The tips of these plates give support to a spherical mass of agglutinated 

 otoliths which consist of Calcium phosphate. Rows of similar but small 

 plates, the four nerves or ciliated furrows, diverge outwards and inter- 



1 The ' stomachal ' plane of Chun corresponds to the ' coeliac ' plane of Agassiz, the ' sagittal ' 

 plane of Claus, Haeckel, and Hertwig, and the ' transverse ' plane of Fol. Chun's ' funnel ' plane is 

 the ' diacoeliac ' plane of Agassiz, the ' transverse ' of Claus and Hertwig, the lateral ' of Haeckel, 

 and the ' tentacular ' of Fol. 



