24 THE HYDROMEDUSAE 



(Plumularia), or both sides ; in the latter case, the hydroids lie in 

 pairs, right and left, opposite to one another (Diphasia), or lie 

 singly, alternating on right and left sides (Obelia). They are 

 stalked (Obelia), or sessile on the hydrocaulus (Plumularia). The 

 perisarc of hydrocaulus and hydrotheca may be either smooth or 

 annulated. While in the Anthomedusae the topmost hydroid of a 

 stem or branch is the oldest, and buds the remainder of the hydroids 

 (or branches) laterally, in the Leptomedusae the formation of new 

 hydroids takes place at the apex of each branch, and the topmost 

 hydroid is therefore the youngest. 



POLYMORPHIC MODIFICATIONS OF THE HYDROID. A blastostyle is 

 very commonly developed, which is devoid of .mouth and tentacles ; 

 the ectoderm at its apex is generally thickened into a special 

 organ, the opercular plate (Figs. 35, 36). Dactylozooids are re- 

 presented in this group by the nematophores (machopolyps), specially 

 modified hydroids provided with hydrothecae (Fig. 30). They are 

 occasionally much elongated and capitate, growing out for some 

 distance from hydrocaulus and hydrorhiza (Ophiodes) ; but more 

 commonly they are short and nearly sessile. In many cases two 

 are placed above each hydroid, and one below it (Aglaophenia, 

 Fig. 31); sometimes they are scattered without symmetrical refer- 

 ence to the hydroids (Plumularia), and are often specially concen- 

 trated round the gonophores (Aglaophenia). They are tentacle-like, 

 with a solid endodermal axis, and are generally provided with a 

 capitulum of cnidoblasts (Plumularia) ; but in some cases the 

 cnidoblasts are replaced by cells which secrete adhesive globules. 

 When both cnidoblasts and adhesive globules occur in the same 

 species, only the nematophores with cnidoblasts are advanced when 

 the colony is disturbed ; when undisturbed only those which form 

 adhesive globules are protruded (von Lendenleld, 33 ; Wagner, 29). 



The MEDUSOID (Figs. 32, 33) is generally much flatter than 

 in the Anthomedusae, its bell less rigid, and its velum smaller 

 and weaker. The manubrium is generally short, rarely absent 

 (Staurostoma) ; the mouth is usually four-lobed, but very numerous 

 accessory lobes are sometimes developed (Polycanna). 



The marginal tentacles are as a rule hollow, rarely solid (Obelia). 

 There may be but two of them (Dissonema) ; generally they are 

 four in number, and perradial in position, or become eight in 

 number on the addition of four interradial tentacles (Eucope). 

 They may increase in number with radial regularity until they 

 amount to hundreds (Aequorea). Their bases are usually expanded 

 into a bulb like those of Anthomedusae. 



Among the marginal tentacles are placed in many cases 

 marginal cirrhi, which are shorter than the tentacles, and have a 

 solid endodermal axis (Laodice). They are highly contractile, 

 often coiled spirally, and provided with a terminal battery of 



