THE CTENOPHORA 



enteron, though the process of digestion is, for the most part, 

 carried on in the stomodaeum, which is provided in its upper 

 portion with a pair of longitudinal thickenings, the stomodseal 

 folds, serving to increase its surface. The products of digestion 

 pass into the infundibulum, and are thence distributed to all parts 

 of the body by canals which, taken collectively, constitute the 

 gastrovascular system. The gastrovascular canals, like the infundi- 

 bulum, are lined with endoderm. 



We may conveniently distinguish two sets of canals vertical 

 and horizontal. The vertical canals consist of a pair running 

 mouthwards, and a single axial vessel passing towards the aboral 

 pole. The former are blind diverticula running down, one on each 

 flattened side of the stomodaeum, and ending in the neighbour- 

 hood of the mouth (Fig. I. 1, 2, stc). The aboral vessel runs 

 straight towards the sense organ, bifurcates at a short distance 

 below it, and each branch again divides to form a pair of small 

 sacs or ampullae which lie immediately below the ectoderm, and 

 underneath the aboral sense organ. Each of the ampullae lies 

 in one of the angles formed by the intersection of the sagittal and 

 transverse planes. Two of them are closed sacs, but two, lying 

 diagonally opposite to one another, open to the surface by small 

 pores in the neighbourhood of the polar fields. It is a rule, with- 

 out exception, in the Ctenophora that, if the animal is viewed from 

 the sagittal aspect, the ampulla farthest from the spectator on the 

 left, and the one nearest to him on the right, open by these so-called 

 excretory pores (Fig. I. 4, and Fig. II. 1, exp). 



The horizontal gastrovascular canals serve to place the infundi- 

 bulum in connection with the bases of the tentacles, and with the 

 eight meridional canals which run immediately beneath the costae. 

 A single pair of wide vessels, lying in the transverse plane, starts 

 from the infundibulum at the level of its opening into the stomo- 

 daeum. Each transverse vessel, after a short course, bifurcates at 

 a wide angle, and its branches again divide, forming on either 

 side of the body four canals, two of which are sub-sagittal and two 

 sub-transverse (Fig. I. 3, 5). Each canal passes direct to a costa, 

 and beneath it is produced orally and aborally into a long diver- 

 ticulum which lies immediately below the costa and ends blindly, 

 forming the sub-costal meridional canal. The gonads are developed 

 on the walls of these sub-costal canals. 



The space between the stomodaeum, gastrovascular system, 

 and body walls is occupied by a gelatinoid substance, in which are 

 imbedded numerous muscle fibres, whose structure and arrange- 

 ment will be described further on. 



The sensory organ at the aboral pole consists of a shallow de- 

 pression of the ectoderm, lined by a modified and probably sensory 

 epithelium. Within many of the epithelial cells are formed cal- 



