60 



COELENTERATA 



[CH. 



importance are that there is a horny shell, the perisarc (Gr. 

 around ; o-a/>, flesh) (Fig. 24), secreted by the ectoderm at any rate 

 on the lower portion, of the body, also that the tentacles are nearly 

 always solid, containing, instead of tubular outgrowths of the endo- 

 derm, a solid cord of cells (Fig. 24) with firm outer membranes and 

 partially fluid contents, so that the cells have the same kind of 

 stiffness as a well-filled water-pillow. These cords likewise grow out 

 from the endoderm, but, as apparently the animal does not need the 

 tentacle cavity which exists in the Hydra, it has disappeared, and 

 the solid axis is essentially a strengthening or skeletal structure. 

 As in Hydra, there is an oral cone; and in some species of 

 Hydro medusae, at any rate, there is an additional row of short 

 tentacles at the tip of this. It has been stated above that the 

 buds do not become detached, but there is one kind of bud differing 

 much in shape from the rest which does become detached. In 

 such a bud, the whole body becomes very much shorter 

 and at the same time much flattened out in its lower 

 portion, so that the main circle of tentacles is widely separated 

 from the oral cone ; at the apex of the latter there is sometimes a 

 second circle of small tentacles. The bases of the longer tentacles 

 which spring from the flattened part of the body are connected 

 together by a web of skin, which constitutes in this way an 



umbrella or bell. The endoderm al 

 canals of the tentacles within 

 this web are termed radial canals. 

 The radial canals are at first 

 quite separate from one another, 

 but they soon acquire broad fringes 

 of endoderm at their sides, and 

 these unite with those of adjacent 

 canals so as to form a continuous 

 sheet of endoderm, the endoderm 

 lamella. The radial canal branches 

 within this lamella and some branches 

 meet those of adjacent canals and 

 form a circular or marginal canal. 

 Other branches lead into extra ten- 

 tacles so that in Obelia there may 

 be a large number of tentacles at 



the edge of the umbrella although there are only four radial canals. 

 In other species the young bud has the same number of tentacles as 



FIG. 25 



Free-swimming Medusa 

 of Obelia sp. 



Mouth at end of manubrium. 

 Tentacles. 



Keproductive organs. 

 Radial canals. 

 Auditory organ. 



