78 POPULAR ILLUSTRATIONS OF 



The neck is very narrow ; and when the animal becomes weak in 

 the aquarium the head readily drops off. In scientific language 

 the head (Fig. 99, d, p. 74) is called a polypite, while that in the 

 tubes is denominated a coenosarc (koinos, common sarx, flesh) 

 (Fig. 99, /, p. 74) ; the two together are called hydrosoma (hydra 

 and soma, body), and the point of attachment or root hydrorhiza 

 (hydra and rhiza, root), while the polyp cells which are absent in 

 Corynidae viz., the small cups in which the polypite lives, are 

 called hydrothecce (hydra and theca, covering). 



These names are all expressive and simple enough. Then the 

 horny or calcareous skeleton was called by Kirby a polypary ; 

 others, Johnston among them, have used the term polypidom to 

 express the same thing ; and the cavity which exists in the body 

 of the polyp, and which extends through the coanosarc, is called 

 the somatic cavity. A little attention to the terminology and the 

 student will be able to pursue his sea-side investigations very easily. 



. CHAPTEE II. 

 THE HYDROZOA (continued). 



The Sertularidce (sertulum, a garland of flowers ; terminal, ides) 

 form the third group into which the Hydrozoa are divided. They 

 are at once distinguished from the Corynidae by the bell-like expan- 

 sions called, it will be remembered, hydrothecae which crowd all 

 the branches of the Zoophyte. Into these bells the expanded head 

 of the polyp can retract, thus distinguishing them from the Cory- 

 nidae, which have not that power. The family is divided into many 

 different genera, and forms by far the most numerous of any of the 

 divisions of Hydrozoa. Fig. 101 represents one of the common 

 species, and may be taken to illustrate the typical parts of the 

 Zoophyte. To show this clearly, I have at Fig. 102 given a repre- 

 sentation of the upper part magnified. The head of the polyp, 

 with all its tentacles spread out in search of food, is seen at a, and 

 at b it is shown contracted and withdrawn into the hydrotheca (c). 

 Then observe the coenosarc (d) passing along from the head (a), 

 covered with a bead-like tube, which, it will be remembered, is part 

 of what is called the polypidom. 



Observe how this coenosarc joins with the other branches, and 

 at e constitutes the centre of the stem of the Zoophyte, just like 

 the pith of a tree. 



