iv PHYLUM CCELENTERATA 149 



they are knobbed at the ends, the knobs being loaded with stinging- 

 capsules (4). 



In some species a- dimorphism of the hydranths obtains, some 

 of them being modified to form protective zooids. In Hydractinia 

 (7) these are simply mouthless hydranths with very short tentacles 

 abundantly supplied with nematocysts, capable of very active 

 movements, and called dactylozooids (dz). In Plumularia there are 

 small structures called "guard-polypes," resembling tentacles in 

 structure, with very numerous nematocysts, and each enclosed 

 in a theca. In Hydractinia the coenosarc is also produced into 

 spines (sp), which may be much modified zooids. 



But the most remarkable modifications occur in the repro- 

 ductive zooids. In a large proportion of genera, both of 

 Anthomedusae and Leptomedusse, these take the form of locomotive 

 medusae, agreeing in general structure with the descriptions 

 already given. Each appears at first as a hollow bud-like process 

 of the blastostyle, or of an ordinary polype, or, more exceptionally, 

 of the coenosarc. This becomes constricted at the junction and 

 rounded off. The ectoderm at its free extremity becomes 

 thickened, and this thickening, as it grows, pushes the endoderm 

 before it, producing a sort of involution. In the interior of the 

 mass of ectoderm a cavity appears : this is destined to form the 

 sub-umbrellar cavity. The ectodermal partition that at first 

 separates the cavity from the exterior, becomes perforated and 

 most of it is absorbed, what remains round the edge going 

 to form the velum. The endoderm is reduced to a thin layer 

 except along four radial lines where it gives rise to the four 

 radial canals, the thin parts between going to form the endoderm 

 lamella. 



In different families and genera the medusae exhibit almost end- 

 less variety in detail. As to size they vary from about 1 mm. in 

 diameter up to 400 mm. (16 inches). The number of tentacles 

 may be very great (Fig. 109, 2) or these organs may be reduced to 

 two (Fig. 109, 7), or even to one (Fig. 105, 3) ; in the last-named 

 cases it will be noticed that the medusa is no longer radially, but 

 bilaterally symmetrical, i.e. it can be divided into two equal and 

 similar halves by a single plane only viz., the plane passing 

 through the one or two tentacles. With the increase in the 

 number of the tentacles a corresponding increase in that of the 

 radial canals often takes place (Fig. 109, J). 



Some medusae creep over submarine surfaces, walking on the 

 tips of their peculiarly modified tentacles (Fig. 105, 6) but the 

 majority propel themselves through the water in a series of jerks 

 by alternately contracting and expanding the umbrella, and so, 

 by rhythmically driving out the contained water, moving with 

 the apex foremost. In correspondence with these energetic move- 

 ments there is a great development of both muscular and 



