30 



INVERTEBRATE MORPHOLOGY. 



through the water with a rotatory motion. The rotation is 

 around a definite axis, one portion of the spherical colony 

 always being in front in progression, and it is noticeable that 

 the stigmata of the individuals of this anterior hemisphere 

 are slightly larger than those of the cells of the posterior 

 hemisphere, a slight differentiation of the individuals being 

 thus present. 



2. Order Dinoflagellata. ■ 



The Dinoflagellata are distinguished from the members of 

 the preceding order by the almost general occurrence of a 

 rather dense shell composed of plates of a substance resem- 

 bling closely vegetable cellulose. Some of the forms, such as 



Geratium (Fig. 14), present a rather 

 bizarre shape on account of the shell 

 being prolonged into horns, and in the 

 majority the shell-plates are delicately 

 sculptured, while around the equator 

 of the shell runs a furrow, and from 

 an opening in the line of the furrow 

 two flagella protrude, one of which 

 possesses the ordinary whip-like char- 

 acter, while the other lies in the fur- 

 row and in some cases has the form 

 Fig. li.—Ceratium tripos of a delicate undulating baud. Ohio- 

 b7BT™r°' '"°"'"" rophyll-like pigment is almost invari- 

 ably present, as is also the red stigma. 

 Peculiar cysts are also present in the protoplasm of many 

 forms, consisting of a hollow capsule having rolled up within 

 it a hollow thread, which on occasion may be rapidly evagi- 

 nated and no doubt has a protective function, resembling very 

 closely in its structure the nematocysts of the Coelenterates. 



3. Order Cystoflagellata. 



The order of the Cystoflagellata includes only two o-enera 

 Noctiluca and Leptodiscus. The latter is a somewhat disk -like 

 structure nearly 2 mm. in diameter, while Noctiluca (Fig. 15) 

 is almost globular with a slight depression at one point where 



