ALIMENTARY CANAL OF CCELENTERATA. 



117 



oesophagus. The canals thus appear to be chambers (c) attached to 

 the oesophagus, which unite into a common central space the digestive 

 cavity, or stomach (B), and so communicate with the oesophagus. The 

 number of these chambers is eight in the Octactiniae, and varies 

 in the other Anthozoa, but is arranged according to the same law 

 of numbers, as is expressed in the other characters of their organisa- 

 tion, as for instance in the number of the tentacles. The septa of 

 the gastrovascular system are usually continued for some distance 

 along the wall of the digestive cavity, and terminate as elongated 

 bands or pads. When, therefore, the stock is calcified interradial 

 lamellae are formed, passing inwards from the wall between the 

 gastric lamellae. 



In the colonial Anthozoa, the central cavity is connected in each 

 person by means of a canal system which traverses the coenenchyma 

 (Fig. 45), and thus every individual is directly connected with the 

 rest. This canal system forms a network of tubes of various widths 

 which distribute the nutritive fluid in the stock. At one point of 

 the common trunk, in the stocks of the Octactinia3, several canals 

 are united into a wider space, from which an orifice leads to the 

 exterior; this, probably, serves as a means of regulating the ingress 

 and egress of the water which flows through the gastrovascular 

 system (Pennatula, Renilla). A similar opening has been 

 observed in Cereanthus ; it corresponds to the pore of the Hydrae, 

 and like it is placed at the aboral end of the body. These arrange- 

 ments, which give to the gastric system the significance of a water 

 vascular system, have, in many Anthozoa (Corals), the form of fine 

 pores scattered over the surface of the 

 body; they can only be perceived at the 

 moment they are in function — that is, when 

 expelling water. Similar pores are also 

 found on the tips of the tentacles in many 

 Actinias, etc. All these arrangements call 

 to mind the dermal pores of the Spongiae. 



In the Pennatulidae and Alcyonidae (Sar- 

 cophyton) some, and at times many, per- 

 sons in a colony are less well-developed, and 

 seem to have lost the function of ingesting 

 food. It is not known whether they have 

 any share in the taking in of water. 



§92. 



Fig. 46. The gastrovascular 

 system of a Cydippe. A 

 Lateral view ; the mouth 

 turned upwards. B Seen 

 from the oral pole. 



Iii the Ctenophora, the nutrient cavitary 

 system differs in details only. A stomach, 

 which is very wide in the Beroidae, and nar- 

 rower in the rest, is sunk in the body along 

 its longitudinal axis ; it passes into a space 



which is known as the "funnel/' by means of a narrow canal, which 

 can be closed by muscles. Radial canals (Fig. 46) pass out from the 

 funnel and run along the ciliated ribs or " ctenophores." The radial 



