90 COMPARATIVE ANATOMY CHAP. 



about by strands of longitudinal muscle fibrillse which run in their 

 solid axes and which may be partially striated. Longitudinal and 

 circular muscle fibres have also been observed in the walls of the gastro- 

 canals. 



The Velum of the Cmspedote Medusa (Fig. 64, B, v, p. 73 ; Fig. 65, 

 p. 74 ; Fig. 72, #, p. 95) is a thin membrane which projects from the 

 margin of the disc like a diaphragm into the subumbrellar cavity. 

 The gastro-canal system is never continued into this membrane, which 

 consists of the following portions : 



1. A continuation of the epithelium of the exumbrella. 



2. A continuation of the epithelium of the subumbrella. These two 

 epithelia coalesce at the free inner edge of the velum. Under the former 

 lies a thin supporting lamella, the continuation of the disc jelly ; under 

 the latter, a layer of ectodermal circular muscle fibres (m), a continuation 

 of the circular musculature of the subumbrella. 



V. Tentacles of the Cnidaria, Marginal Lobes of the 

 Seyphomedusae. 



All Cnidaria (with the exception of the Rhizostomce among the 

 Scyphomedusce and the Amalthceidce among the Craspedota) possess 

 tentacles arranged in a circle round the mouth, at a greater or less 

 distance from it. These tentacles are evaginations of the body wall, 

 into which (with the perhaps only apparent exception of the Ctenophora) 

 hollow or solid processes of the endodermal gastro-canal system pene- 

 trate. The tentacles are pre-eminently organs for catching food, and 

 at the same time sensory organs of touch. We shall see further on 

 that some of them are partially transformed in the Craspedote and 

 Acraspede Medusce into specific sensory organs. 



The structure of the tentacles, their number, their division and 

 arrangement on the body, and their relation to the gastro-canal system, 

 offer in the various divisions many modifications of great importance 

 in classification. 



Form of the tentacle. The tentacles are, speaking generally, 

 cylindrical filaments. In the Hydroids they are usually simply fila- 

 mentous, less frequently knobbed at the free end, and still less fre- 

 quently branched (Cladocoryne). Among the Craspedote Medusce also 

 we generally meet with filamentous tentacles. The family of the 

 Cladonemidce alone is distinguished by tentacles which are dichoto- 

 mously branched, or feathered on one side (provided with collateral 

 filaments), and often knobbed. The tentacles of the Siplionopliora and 

 the Ctenophora are also feathered on one side. The tentacles of the 

 Acraspeda are simple. Among the Antlwzoa, the Alcyonaria possess 

 tentacles feathered in two rows, but all other divisions have simple 

 filamentous or vermiform tentacles. 



Number and arrangement of the tentacles. Among the Hydroida 



