iv PHYLUM COELENTERATA 149 



Protohydra (Fig. 109) and Microhydm, in which the tentacles 

 are absent. 



Pelagohydra is also solitary, but is pelagic. The part corre- 

 sponding to the base in Hydra here takes the form of a float, and 

 there are tentacles distributed over the surface of the float as well as 

 in the neighbourhood of the mouth ; medusae are developed from 

 processes on the float. Pelagohydra, however, is perhaps more 

 nearly related to the Siphonophora an order yet to be dealt with 

 than to the Leptolinae. 



The polypes are usually cylindrical, as in Obelia, but in some 

 genera they are widened out into a vase-like form (Fig. 106, <5), in 

 others elongated into a spindle-shape (4). The tentacles may be 

 disposed in a single circlet, as in Obelia and Hydra, or there may 

 be an additional circlet round the hypostome (3, <5), or at the base of] 

 the polype, or they may be scattered irregularly over the whole j 

 surface (4). In Myriothela (2) they are short, and so numerous- 

 as to have the appearance of close-set papillae. In some forms 

 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 

 (1) 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 

 Anthomedusse and Leptomedusae, 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 



