ROTATORIA. 245 
cesophageal about the middle of the body, long commissures between 
them, and numerous nerves from both; it retains its primitive con- 
nection with the epidermis. There are two eyes and various patches 
of sensitive cells. The food canal is complete and simple, and lies in a 
spacious ciliated body cavity. Corresponding to the external divisions, 
the cavities-of the head, body, and tail are distinct, being separated 
from one another by septa; a longitudinal mesentery supports the gut 
and divides the cavities into lateral halves, 
There is no vascular system, nor are there any certain nephridia. It 
is possible that the latter may be represented by the genital ducts. 
_ The animals are hermaphrodite, and the simple reproductive organs 
lie near one another posteriorly. The two ovaries project into the 
body cavity, and their ducts open laterally where body and tail meet. . 
The two testes project into the cavity of the tail ; and their ducts have 
Fic. 126.—Development of Sagztfa.—After O. Hertwig. 
Illustrating formation of a body cavity by pockets 
from the archenteron; also the early separation of 
reproductive cells. 
£c., Ectoderm; £z., endoderm; ac., archenteron; &., repro- 
ductive cells; 42, blastopore; ¢.g., ccelom pouches; ™., 
mouth; x. section of gastrula; 2 and 3. origin of celom 
pouches. 
internal ciliated funnels, and open on the tail. Two reproductive cells 
are set apart at 4 very early stage, and each divides into the rudiment 
of an ovary and ofa testis. The eggs undergo complete segmentation ; 
a gastrula is formed by the invagination of the blastula; the body 
cavity arises, in enteroccelic fashion, as two pockets from the arch- 
enteron. The young forms are like the adults. 
Appendix (2) to Annelid Sertes 
Class Rotatoria. Rotifers 
Rotifers are beautiful minute animals, abundant in fresh water, also 
found in damp moss, and in the sea. They owe their name and the 
old-fashioned title of wheel-animalcules to the fact that the rapid move- 
