THE ANIMAL KINGDOM 411 



Physiologically, rotifers have remarkable powers of resistance 

 to heat and drying. Some species when slowly dried on a 

 slide, secrete a protective covering that enables them to survive 

 lack of water and extremes of temperature. Such animals can 

 be revived if placed in water after a year or more of desiccation. 

 Under natural conditions this property enables them to survive 

 drought and unfavorable conditions of temperature. 



Example: Hydatina senta, a fresh-water species. 

 CLASS II. GASTROTRICHA. Gastrotichs are common 

 aquatic forms but less well known than rotifers, from which 

 they differ principally in possessing cilia on the flattened ventral 

 surface of the body. The head is characteristically marked off 

 from the trunk by a narrow neck region. They are both dioe- 

 cious and monoecious. Some of them measure only 1.5 mm. in 

 length. 



Example: Lepidoderma rhomboides, a fresh-water species. 

 CLASS III. KINORHYNCHA. Very small marine trochel- 

 minthes measuring from 0.18 to 1.0 mm. in length; lacking cilia 

 but provided with spines and bristles. Locomotion char- 

 acterized by a peculiar invagination and evagination of the 

 head. Sexes are separate. 



Example: Trachydemus mainensis, found on tidal flats, 

 Maine coast. 



PHYLUM 9— BRYOZOA 



Bryozoa (moss animals) are small, usually colonial animals, 

 found on the surface of rocks, plants, and other objects in salt or 

 fresh water. Externally some of them bear a resemblance to 

 compound hydroids, but internally the resemblance ceases; for the 

 individual zooids have a body cavity and a complete alimentary 

 canal, bent so that the anus lies near the mouth (Fig. 239) . There 

 is usually a central nervous system between the mouth and the 

 anus, and in some a pair of nephridial tubes with flame cells. The 

 lophophore is a characteristic horseshoe-shaped ridge about 

 the mouth, bearing hollow, ciliated tentacles. The epidermis 

 (hypodermis) secretes a calcareous cuticle, the ectocyst, which 

 protects the soft parts of the animal. In some the tentacles can 

 be completely withdrawn into the ectocyst. These animals are 

 either monoecious or dioecious. They are very ancient forms of 

 life, fossils having been found in the Cambrian and all subsequent 

 formations. The great majority of the species is marine. 



