HYDROSOME AND MEDUSOME 



251 



consists of a branched tube opening by mouths at the ends of 

 the branches and closed at the base. The body-wall is built 

 up of ectoderm and endoderm. Between these layers there is 

 a thin non-cellular lamella, the mesogloea. 



In a great many Hydrozoa the ectoderm secretes a chitinous 

 protective tube called the " perisarc." The mouth is usually a 

 small round aperture situated on the summit of the hypostome, 

 and at the base of the hypostome there may be one or two 

 crowns of tentacles or an area bearing 

 irregularly scattered tentacles. The 

 tentacles may be hollow, containing a 

 cavity continuous with the coelenteric 

 cavity of the body; or solid, the endo- 

 derm cells arranged in a single row 

 forming an axial support for the ecto- 

 derm. The ectoderm' of the tentacles is 

 provided with numerous nematocysts, 

 usually arranged in groups or clusters 

 on the distal two-thirds of their length, 

 but sometimes confined to a cap -like 

 swelling at the extremity (capitate ten- 

 tacles). The hydrosome may be a single 

 zooid producing others asexually by 

 gemmation (or more rarely by fission), 

 which become free from the parent, or 

 it may be a colony of zooids in organic 

 connexion with one another formed by 

 the continuous gemmation of the original 

 zooid derived from the fertilised ovum 

 and its asexually produced offspring. 

 When the hydrosome is a colony of zooids, specialisation of 

 certain individuals for particular functions may occur, and the 

 colony becomes dimorphic or polymorphic. 



The medusome is more complicated in structure than the 

 hydrosome, as it is adapted to the more varied conditions of a 

 free-swimming existence. The body is expanded to form a disc, 

 " umbrella," or bell, which bears at the edge or margin a number 

 of tentacles. The mouth is situated on the end of a hypostome, 

 called the " manubrium," situated in the centre of the radially 

 symmetrical body. The surface that bears the manubrium is 



Fio. 125. — Diagram of a ver- 

 tical section through a 

 hydrosome. God, Coelen- 

 teron ; Jict. ectoderm ; Jind, 

 endoderm. Between the 

 ectoderm and the endoderm 

 there is a thin mesogloea 

 not represented in the 

 diagram . M, mouth ; T, 

 tentacle. 



