206 



LOWER INVERTEBRATES. 



The rotifers, like a few others of the lower animals, have an almost incredible 

 power of withstanding desiccation. Apparently, in the most unfavorable conditions of • 



deficient moisture, evaporation from their 

 bodies does not proceed further than to 

 interrupt their activity without destroying 

 their vitality. Apparently they may exist 

 in this dried condition 'for an indefinite 

 period, and resume their course of life upon 

 being again sujDplied with water. " This 

 fact, taken in connection with the extraor- 

 dinary rate of increase mentioned in the 

 preceding chapter, removes all difiiculty in 

 accounting for the extent of the diffusion 

 of these aniuials, and for their occurrence 

 in incalculable numbers in situations where, 

 a few days previously, none were known 

 to exist ; for their entire bodies may be 

 wafted in a dried state by the atmosphere 

 from place to place," and they return to 

 active life whenever they encounter neces- 

 FiG.i9i.~irj/datinasenta; ^.female; ^, male. sary conditions of moisture and warmth. 



C1.AS8 m. — GASTROTRICHA. 



These minute worms, barely more than microscopic in size, are 

 found in quiet waters in company with other animalcules. They owe 

 their name to the presence of locomotive cilia upon 

 the under surface of their bodies. The number of 

 species is limited, and there have been thus far only 

 seven genera established, of which Icthyditwi and 

 Clio-tonotus are the most common. The body is 

 flask-shaped, and terminates in a fork formed by two 

 short processes. The mouth lies at the front ex- 

 tremity, and leads into the very noticeable muscular 

 oesophagus. There is usually a pair of pigmented 

 eye-specks in front. All the species are probably 

 hermaphroditic. Beyond these few anatomical de- 

 tails we have little knowledge about the group — 

 nevertheless it is particularly interesting because the 

 type of oi'ganization is extremely simple, indeed, I 

 think the simplest of all the animals with an organ- 

 ization essentially bilaterally symmetrical. We hope 

 that some patient investigator will soon work out the 

 development and entire life history of these worms. 

 Closely related to the Gastrotricha, and yet offering striking pecu- 

 liarities of its own, is a small marine worm, Echinoderes, which was 

 first discovered at Saint Malo, in 1841, by Dujardin. The body is 

 from a third to a half of a millimetre long. The shape of the animal. 



Fig. 192.— IcIUIij/dium. 



\ 



\ 



. — Echijw- 



