92 COMPARATIVE ANATOMY CHAP. 



tissue. Under the outer epithelium lies an ectodermal muscle layer, 

 and under the inner epithelium a similar endodermal layer. In some 

 cases muscles may also run within the supporting substance. 



The tentacles of the Ctenophora are solid. They are generally 

 provided with seizing or adhesive cells. Their axes are usually occupied 

 by strongly developed longitudinal muscle-fibres. These fibres arise, 

 as it appears, at an early stage out of special " mesodermal " elements, 

 i.e. a group of cells which sever themselves from the primitive endo- 

 derm of the young gastrula larva. We ought therefore, perhaps, to 

 compare the solid axis of the Ctenoplwmn tentacle with the solid endo- 

 dermal axis of the tentacles of many Medusce. 



The marginal lobes of the Seyphomedusse (Fig. 67, rl, p. 77 ; 

 Fig. 70). The marginal lobes of the Scyphomedusce or Acraspeda are 

 just as characteristic of them as is the velum of the Hydromedusce or 

 Craspedota. As a real velum is wanting in all Scyphomedusce, so are 

 marginal lobes wanting in all Hydromedusce. Like the tentacles, 

 together with which they are found, the marginal lobes are processes 

 of the body wall at the edge of the umbrella, into which the prolonga- 

 tions of the gastro-canal system extend. Unlike the tentacles, they 

 are broad and flat, serving as rowing organs, with muscles on the con- 

 cave subumbrellar side. In the simplest cases there are 8 adradial 

 marginal lobes, mostly, however, there are 16 subradial lobes, and 

 their numbers are often still further increased. In the Cubomedusce 

 and in many Rhizostomce the lobes grow together to form a circular 

 rim of varying width, the so-called velarium which, however, can 

 always be easily distinguished from the true velum of the Craspedota 

 by its supply of gastro- canals. The clusters of tentacles of the 

 Lucernaria are secondary outgrowths of the marginal lobes, between 

 which rudiments of the primary tentacles can sometimes be found. 



VI. The Nervous System. 



This, which appears in the Cnidaria for the first time in the 

 animal kingdom as an independent system, is marked by a rather 

 diffuse arrangement (want of definite centralisation), and by the close 

 relation which it bears to the body epithelium during the whole life. 

 In the Hydra the outwardly directed cell body of the so-called neuro- 

 muscular cells probably plays the part of an undifferentiated sensory 

 nerve cell. But as early as the Craspedote Medusce we find an inde- 

 pendently developed nervous system, close under the epithelium, outside 

 the supporting membrane or jelly; it forms a plexus of bipolar and 

 multipolar ganglion cells with connecting fibrillse. This plexus is con- 

 nected by fibrillse on the one hand with the epithelial sensory cells 

 (tactile, auditory, visual, olfactory cells), on the other hand with 

 muscle fibres. In correspondence with this, the nervous tissue is 

 particularly strongly developed on the margin of the umbrella, which 





