ii GNIDARIA GENERAL 75 



From the margin of the disc a thin muscular membrane projects 

 into the subumbrellar cavity like a diaphragm ; it is known as the 

 Velum (r), and is one of the principal locomotory organs of the 

 Craspedote Medusce. 



There are many other organs at the margin of the disc, the princi- 

 pal being : 



1. A double nerve ring. 



2. Marginal vesicles. 



3. Eye spots. 



4. A ring of nematoeysts. 



We shall speak of these organs later on. 



The derivation of the Medusa form from the Hydropolyp is the 

 more probable as Medusce belonging to many divisions, in their in- 

 dividual development, proceed from Hydropolyps by lateral budding. 

 Such AL-diiw are then specially developed buds which have the 

 function of forming the sexual products and of scattering them abroad. 



2d Form : the Scyphopolyp or the Scyphula (type : Lucernaria or 

 a simple Coral individual) is the original form of the Scyphozoa, as 

 the Hydropolyp is of the Hydrozoa. It is distinguished from the 

 Hydmpiliips in that at the oral pole the region about the mouth sinks 

 in to form an cesophageal tube (Fig. 99, p. 130). The epithelium 

 which lines it is thus ectodermal. Around this tube the gastric cavity 

 falls into 4, 6, or 8 compartments separated by walls ; these partition 

 walls, whose free edges project into the gastric cavity as septa or 

 tsenioles, bearing either mesenterial thickenings or gastral filaments, 

 are continued along the body Avail even to the aboral end of the body. 

 All Corals remain essentially at the stage of the Scyphopolyps (Fig. 

 66 ; Fig. 82, p. 107). Their typical form is that of the cylinder 

 or reversed truncated cone. The two ends of the body are almost- 

 circular ; by the aboral pedal disc the animal attaches itself, and 

 hollow tentacles stand round the oral disc (especially at the edge). 

 In the middle of the oral disc lies the generally elongated slit-like 

 external mouth. 



The external mouth, in keeping with the Scyphopolyp plan, leads, 

 not directly into the gastric cavity, which is lined with endoderm, but 

 into a tube which has arisen by an invagination from without, and is 

 therefore lined with ectoderm. This tube (cesophageal tube, stomo- 

 daeum) is open at both ends. One opening, the external mouth, leads 

 to the exterior; the other (the enteric aperture) leads into the spacious 

 gastric cavity. 



The body wall consists of ectoderm and endoderm, between which 

 a solid middle layer is interposed, of which we shall speak later. The 

 endoderm and the middle layer raise themselves from the body walls, 

 forming vertical ridges which project into the gastric cavity through- 

 out the whole length of the body ; these ridges are the septa. In the 

 neighbourhood of the cesophageal tube these septa project far enough 



