148 ZOOLOGY 



SECT. 



body with a conical hypostome and a circlet of six or eight 

 tentacles. It is ordinarily attached, by virtue of a sticky secretion 

 from the proximal end, to weeds, &c., but is capable of detaching 

 itself and moving from place to place after the manner of a loop- 

 ing caterpillar. The tentacles are hollow, and communicate freely 

 with the enteron. Both the body and the tentacles are highly 

 contractile, the contractions being effected by means of a layer of 

 fibres which run longitudinally. These fibres are processes the 

 muscle processes (C, m. pr.) of the large ectoderm cells. Similar 

 shorter muscle-processes of some of the endoderm cells run 

 circularly and antagonise the longitudinal fibres. Nematocysts are 

 abundant in the ectoderm. The endoderm cells are mostly 

 amoeboid and vacuolated. Each usually bears one or more flagella, 

 but these may be retracted. Glandular cells occur here and there. 

 Nerve-cells (multipolar) occur in both layers, but present no regular 

 arrangement. There is no perisarc. Buds (bd. 1, bd. 2) are 

 produced which develop into Hydrae, but these are always detached 

 sooner or later, so that a permanent colony is never formed. There 



FlG.10S.-Protohydra leuckartii. (From Chun, after Greeff.) The mouth is to the left, the 

 disc of attachment to the right. 



are no special reproductive zooids, but simple ovaries (ovy) and 

 testes (spy) are developed, the former at the proximal, the latter 

 at the distal end of the body. Even simpler than Hydra are 

 Protohydra (Fig. 108) and Microhydra, in which the tentacles are 

 absent. 



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

 ponding 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 Sipkoiwpkora an order yet to be dealt with j 

 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. 105, 5), inj 

 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 

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

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



