INVERTEBRATES OF UNCERTAIN POSITION 183 



carries the eggs (ov) into the cloaca (cl). From here the eggs 

 reach the exterior through the anus. The males are usually 

 smaller than the females, and often degenerate. They possess 

 a testis in which the spermatozoa arise, and a penis for trans- 

 ferring the spermatozoa to the female. 



Two kinds of eggs are produced by rotifers: (1) summer 

 eggs, and (2) winter eggs. The summer eggs, which develop 

 parthenogenetically, are thin-shelled, and of two sizes ; the 

 larger produce females and the smaller males. The winter 

 eggs, which are fertilized, have thick shells, and develop 

 females. 



One peculiarity of the rotifers worth mentioning is their 

 power to resist desiccation. Certain species, if dried slowly, 

 secrete gelatinous envelopes which prevent further dtying; in 

 this condition they live through seasons of drought, and may be 

 subjected to extremes of temperature without perishing. 



The resemblances between rotifers and the trochophore 

 larvae of certain mollusks, annelids, and other animals to be 

 described later, is quite striking. The larva of the Nemertinea 

 {Pilidium, Fig. 118) is likewise similar in some respects to an 

 adult rotifer. This has led to the theory that the rotifers are 

 animals somewhat closely connected with the ancestors of the 

 mollusks, annelids, and certain other groups. 



7. Bryozoa (Polyzoa) 



TheBRYOZOA (Gr. bruon, moss; zo'on, an animal), Phoronidea, 

 and Brachiopoda are sometimes placed together under one 

 phylum, the Molluscoidea, because they are mollusk-like in 

 form. It seems probable, however, that they not only repre- 

 sent distinct, but widely divergent groups, and should therefore 

 be discussed separately. 



The Bryozoa, or moss-animals, are mostly colonial. They 

 resemble hydroids, like Obelia (Fig. 73), in form, but differ from 

 them markedly in structure. The majority of them live in 

 the sea, but a few inhabit fresh water. Bugula (Fig. 124) is 



