the structure and physiology of these marvellous little beings. First 

 is the 



Zooid. — Any of the principal elements, such as the Polypite, or the 

 reproductive body, etc. 



Polypite. — The animal itself, consisting of a digestive sac termi- 

 nating with the mouth, and surrounded with tentacles. Occasionally 

 they are single, as in the Hydra, but generally consist of a large 

 number, comprising the plant-like colony. 



Ccenosarc. — The flesh-like trunk which unites one Polypite to its 

 neighbour, forming a compound Zoophyte. It is divided into two 

 parts — the stem, and the adherent base or stolon. 



Polypary. — The chitinous sheath which invests the ccenosarc or 

 fleshy trunk, in some species only partially. 



Hydrotheca or calycle. — The bell-shaped organ in which the Poly- 

 pite is lodged. It is sometimes called the cell, but as there are so 

 many definition^ of the word cell the term " calycle " is generally 

 used. 



Gonophore. — The bud in which are developed the reproductive 

 organs. It consists of an external sac (ectotheca) covering either a 

 fixed generative sac, in which the ova and spermatozoa are developed, 

 or a free sexual Zooid which is called the 



Goiwzooid. — They are sometimes fixed, at others free. 



Gonotheca or capsule. — The chitinous receptacle or pocket in 

 which the gonophores are developed. They vary much in shape 

 according to the species. 



Umbrella or swimming-bell. — A contractile disc which enables 

 the medusiform Gonozooid to propel itself. 



Manubrium. — The hollow body with its mouth at the extremity. 

 It hangs free from the top of the umbrella, in the position the stick 

 holds in that useful article. 



Velum. — A delicate membrane which partially closes the swim- 

 ming-bell or umbrella. 



Lithocysts. — -Small organs developed on the edge of the swimming- 

 bell — probably organs of sense. 



Planula. — The usual form of the Hydroid embryo. 



Trophosome. — The whole colony of alimentary Zooids comprising 

 the hydroid colony. 



Gonosome. — The sexual Zooids of the community. 



Ectoderm. — Their outer covering. 



Endoderm. — Their inner covering. 



Aematocysts or thread-cells. — Minute sacs containing delicate 

 projectile threads. 



Paplocil. — A rigid, hair-like process occurring on the tentacles of 

 some Hydrozoa, supposed to be organs of touch. 



In the classification, which is somewhat complex, Johnston places 

 the Hydra (a fresh-water species) first on the list. Hincks places it 

 last, in a sub-order (Gymnochora). I think that, as they are devoid 

 of any chitinous covering or polypary, they should come first ; at all 



