COELENTERATA. 



115 



aggregations of nervous tissue in certain regions. In the. craspedote 

 medusae there are two annular nervous tracts in the ectoderm at 

 the base of the velum, one on each side of the supporting lamella 

 (Fig. 102 ri, n"). They consist of ganglion-cells, nerve-fibres,>some 

 of which perforate the supporting lamella to put the two rings in 

 communication, and of sensory epithelial cells bearing stiff projecting 

 sensory hairs, and ending internally in fibres which pass into the 

 nerve-rings. 



In the Acraspeda there is a special aggregation (Fig. 103 F) of the nervous 

 tissue, of the same structure as the nerve-rings of the Craspedota, round the 

 base of each of the sense tentacles (marginal bodies, or rhopalia) ; and in the 

 Charybdeidae there is a sub -um- 

 brella marginal nerve with a zigzag 

 course connecting together these 

 rudimentary ganglia, which in the 

 other forms are apparently inde- 

 pendent of one another. In the 

 Acraspedota, pits the so-called 

 olfactory pits lined by a sensory 

 epithelium are found on the edge 

 of the umbrella above the marginal 

 bodies (Fig. 103 K). 



At the edge of the um- 

 brella are always placed special 

 sense organs the marginal 

 bodies. They are of two 

 kinds those which by their 

 structure indicate an auditory 

 function, and those which 

 indicate a visual purpose. In 

 the craspedote medusae they 

 are mutually exclusive, so that 

 we find ocellate medusae with 

 sense organs of the visual 

 type and vesiculate medusae with sense organs of the auditory type. 

 In the Acraspeda the marginal bodies may carry organs of both 

 types. 



The sense organs of the auditory type always contain concretions the 

 otoliths of an organic or inorganic material, which concretions are contained 

 inside cells ; and two types of auditory organ are distinguished according as 

 these concrements are contained in cells of the ectoderm (Fig. 104) or in cells 

 of the endoderm (Fig. 105). In the Craspedota we find auditory organs of both 

 types. In the Leptomcdusae they consist of ectodermal pits, which may be 

 closed into vesicles, on the under side of the base of the velum ; some of the 



71.0C. 



St. 



FIG. 102. Diagrammatic transverse section 

 through the edge of the umbrella of Carmarina 

 hastata (from Chun), v velum ; $ t supporting 

 lamella of velum ; mu circular muscles of velum 

 in section ; n 1 lower, n" upper nerve ring; n.iv 

 thickening of the ectoderm of the edge of the 

 umbrella ; c.r radial vessel ; r.k circular vessel ; 

 en solid cord of endoderm beneath the peronium 

 pe ; ot tentaculocyst ; n.ac auditory nerve ; ga 

 umbrella jelly. 



