II GNIDABIA— TENTACLES, MARGINAL LOBES 91 



the tentacles, in varying number, are either arranged in a circle at a 

 short distance from the mouth {e.g. Hydra), or in two circles {e.g. 

 Twbularia), or they are dispersed over the body of the individual, 

 though not on its stalk {e.g. Coryne). In the Scyplmtoma the tentacles 

 (about 24 in number) are inserted at the edge of the oral disc of 

 the cup-shaped body. In the Craspedote and Acraspede Medusce the 

 tentacles are invariably found on the margin of the disc. In the 

 simplest case we find in the former 4 perradial, and in the latter 

 4 perradial and 4 interradial tentacles. In most Medusse the 

 number of tentacles increases in a regular manner — secondary, tertiary, 

 etc., being inserted between the primary. Only in a few Craspedota 

 the number is reduced to two tentacles, opposite one another, and less 

 frequently to one single tentacle (in the sub- family of the Euphysidce 

 among the Codonidce, and in the tentacle-bearing '' persons " of one 

 principal division of the Siphonophora (the Siphonantha). Complete loss 

 of all tentacles is a distinguishing mark of the Amalthceidce among 

 Craspedota and the Bhizostomce among Acraspeda. In a few Medusse, 

 especially among the Narcomedusce, the points of insertion of the 

 tentacles move up from the edge of the disc a greater or less distance 

 on to the exumbrella. 



In the Corals the number of tentacles inserted at the circumference 

 of the oral aperture represents, in a general way, the number of gastric 

 pouches separated by septa. Each tentacle lies above a gastric pouch, 

 which is produced into it in the form of an axial canal. In the Ahyo- 

 naria we have 8, in the Hexacwallia 6 or 6n, in the TetracoralUa 4 or 

 4:11, such corresponding tentacles. 



The majority of the CtenopJwra {Tentaculata) possess two solid 

 tentacles or retractile filaments, feathered on one side, lying in the 

 lateral plane ; these can be withdrawn into special sacs or sheaths, and 

 arise in the neighbourhood of the aboral pole ; later, however, they 

 generally move towards the oral pole. 



Structure of the tentacles. — The tentacles of the Hydromedusce 

 and Scyphomedusce consist: (1) of an ectodermal epithelium, generally 

 with stinging knobs or batteries ; (2) of a layer of ectodermal longi- 

 tudinal muscle fibres lying directly under this ; (3) of a structureless 

 elastic supporting membrane ; and (4) of an axis of endodermal cells. 

 This axis is hollow in most forms, and the cavity is in open communi- 

 cation with the gastro-canal system of the body ; or it is solid, and 

 then consists chiefly of a single row of disc-like cells, which are super- 

 imposed like the coins in a roll of sovereigns {e.g. in Obelia, in many 

 TrachomeduscB, Narcomedusce, and the related Hydroids, also in Stauro- 

 medusce and Cannostomce). This axial pillar must serve as an elastic 

 organ of support. The hollow tentacles are mostly far more mobile 

 and more strongly contractile than the solid. The tentacles of Corals 

 are also hollow ; their structure differs considerably from that of the 

 Hydro- and Scypho-medusce. The ectoderm and endoderm in Corals are 

 separated by a supporting substance which contains cells of connective 



