CCELENTERATA : MEDUSID.E. 1 29 



tentacles, which are usually hollow processes, composed of 

 both ectoderm and endoderm, and in immediate connection 

 with the canal system. Also round the circumference of the 



Fig- 53- Trachynema digitale, a naked-eyed Medusa, female, enlarged. (After A. 

 Agassiz.) p Manubrium or central polypite ; t One of the tentacles ; c One of the 

 gastro-vascular canals ; o One of the ovaries. 



nectocalyx are disposed certain " marginal bodies," of which 

 two kinds may be distinguished. Of these the first are termed 

 " vesicles," and consist of rounded sacs lined by epithelium, 

 and containing one or more solid, motionless concretions 

 apparently of carbonate of lime immersed in a transparent 

 fluid. The second class of marginal bodies, variously termed 

 " pigment-spots," " eye-specks," or " ocelli," consists of little 

 aggregations of pigment enclosed in distinct cavities. The 

 "vesicles" are probably rudimentary organs of hearing, and 

 possibly the eye-specks are a rudimentary form of visual ap- 

 paratus. The oral margin of the polypite may be simple, or it 

 may be produced into lobes, which are most frequently four 

 in number. The essential elements of generation are produced 

 in simple expansions either of the wall of the manubrium or of 

 the radiating nectocalycine canals. 



I 



