254 CCELENTERATA. 



The gastrovascular apparatus of the cup- or bell->haped Calycozoa 

 and Charybdeidce differs from the types above described, and re- 

 sembles that of the more primitive Scyphistoma stage, in that the 

 gastric cavity presents only four peripheral vascular pouches, which 

 are very wide, and separated by extremely thin septa. 



The worm-like movable tentacles of the gastric cavity, the gastric 

 filaments, which are not found in any Hydromedusce afford an im- 

 portant distinctive mark. They correspond to the so-called mesenteric 

 filaments of the Anthozoa, and afford the same aid to digestion 

 through the secretion of their glandular entodermal covering. In 

 every case they are attached to the sub-umbrella wall of the 

 stomach, and fall in the four radii of the generative organs (radii of 

 the second order), which alternate with the radii of the angles of the 

 mouth, or radii of the first order. They visually follow the inner 

 edge of the generative organs in a simple or convoluted curved line. 



The existence of the nervous system of the Acalepha has only 

 recently been demonstrated with certainty. It has been proved that 

 the centres of the nervous system are contained in the ectoderm of, 

 the stalk and base of the marginal bodies, and consist of a considerable 

 layer of nerve fibrillse deep in the ciliated ectodermal epithelium, 

 the nerve cells of which are elongated in the form of a rod, and bend 

 round at their basal extremities to be continued directly into the 

 nerve fibrillre (fig. 196). There is in addition a widely distributed and 

 important peripheral nerve plexus in the muscles of the sub-umbrella. 



Up to the present time no investigations have completely elucidated 

 the manner in which this nerve plexus is related to the nerve centres 

 of the marginal bodies, and how the latter are connected with one 

 another. The existence of a nerve ring on the sub-umbrella surface 

 has been proved only for the Charybdeidce, in which the edge of the 

 disc is not notched (fig. 169). The antimeres of the Acaleplm show 

 in all cases a great degree of individuality, and, when cut off, are able 

 to live for a considerable time. 



The marginal bodies, as well as the pit-like depressions on the 

 dorsal side of the excavations in which the marginal bodies are 

 placed (olfactory pits), must be considered as sense-organs. 



The marginal bodies are morphologically the remnants of reduced 

 tentacles. They may be seen on the under side of the umbrella in 

 the stage of the Ephyra, and are overgrown by portions of the edge 

 of the umbrella (Steganophthalmata). [They contain a central canal 

 lined by endoderm and continuous with the gastro-vascular system 

 of the disc, fig. 196]. They appear in all cases to unite the functions 



