Class 1. Hydrozoa. Order 2. Siphonophora. lOQ' 



enclosing numerotis pneumatooysts, and bearing the various individuals on 

 the underside (a large gastrozooid centrally, dactylozooids at the edge, no 

 nectooalyoes or hydrophyllia) ; Velella like the previous foiins, but the disc is. 

 elliptical with an upright keel. All met with in the Mediterranean. 



Order 3. Ac£lleph.a {ScyphomeduscB , Acraspeda). 



The medusoid generation is usually represented by transparent 

 animals of fair size. Tlie edge of the mouth is prolonged into four 

 oral tentacles with which the prey is caught. Similar processes^ 

 but as a rule less well developed^ may occur in the Hydromedusse. 

 The canal in the manubrium widens out in the umbrella to form the 

 gastric cavity, which may extend into a number of large radial out- 

 growths (gastric pouches) : and the radial canals, often branched, also- 

 arise from it. It contains a number of tentacular threads, the gastral 

 filaments, which are absent from the Hydromedusse ; also the gonads, 

 ovaries and testes, usually in the form of four folded bands ;. 



Fig. 64. Section through a Scyphomedusa passing between two oral tentacles, d hood. 

 ooTering one o{ the tentaoulocysts, h, sub-genital pit, m mouth, o ovary, r tentaoulocyst, 

 rk radial canal, t gastric portion of enteric cavity, t' radial portion of ditto, ie gastral 

 filament. — Orig. 



beneath each of these, on the under side of the umbrella, there is a 

 cavity separated from the enteric cavity by a thin septum {h Fig. 64) ;. 

 this is pushed out when the genitalia are well-developed, so that 

 they appear to depend from the oral side of the umbrella. The 

 ova and spermatozoa which usually occur in different individuals, 

 fall into the enteric-cavity, and escape through the mouth. 

 The cells from which they originate, are endodermal. The 

 umbrella in which the mesogleea attains a considerable thickness, 

 has eight notches in its margin; in each notch, covered by 

 a lappet of the umbrella, there is a small round marginal body 

 which bears an audit or y, often also an optic organ. Along 

 the edge of the umbrella, there is often a considerable number 

 of marginal tentacles, usually of a considerable length ; a velum, 

 on the other hand, is wanting. The ova of Aurelia develop, as. 



