PHYLUM COELENTERATA (cNIDARIA). SIMPLE TISSUE ANIMALS 



105 



zoophytes, and a few stony hydroids; (2) 

 the Scyphozoa, mostly large jellyfishes; and 

 (3) the Anthozoa, which include the sea 

 anemones and most of the stony and horny 

 corals. 



HYDRA-A FRESH- 

 WATER HYDROZOAN 



Hydras are simple coelenterates, abun- 

 dant in fresh-water ponds and streams. Nine 

 known species occur in the United States. 

 They are easily seen with the naked eye, 

 are usually 2 to 20 mm. in length, and re- 

 semble a short thread frazzled at the unat- 

 tached distal end. The great variation in 

 length exhibited by hydras at different times 

 is due to the fact that both body and tenta- 

 cles are capable of remarkable expansion 

 and contraction because of specialized con- 

 tractile fibers. The coelenterates we know as 

 hydras were named after the mythological 

 nine-headed dragon slain by Hercules. 



Hydras are of particular interest in that 

 their adult organization corresponds roughly 

 to the gastrula of higher animals. Thus they 

 may be regarded as the living counterparts 

 of some remote ancestor of the higher meta- 

 zoans. They are of further interest in that 

 they exhibit a complex organization in a 

 so-called simple animal, but there is little 

 division of labor. The work performed by 

 organs in higher animals is thrown upon the 

 tissues and individual cells in the hydra. As 

 with most "simple" animals, anyone who 

 studies these cells and tissues carefully 

 comes to realize that the supposed simplicity 

 of the hydra is largely fallacious. 



Gross morphology 



The body of the hydra (Fig. 50) resem- 

 bles an elastic tube which may be extended 

 to a length of 2 cm. At the distal end is a 

 circlet of tentacles, usually 6 or 7, and as 

 many as 10 in some species. Some hydras 

 have extremely extensible tentacles, which 



may stretch out to several times the body 

 length; they have a stalk portion well set off 

 from the rest of the body. 



The tentacles are capable of remarkable 

 extension, and may stretch out from small 

 blunt projections to very thin threads 7 cm. 

 or more in length (Fig. 50). They move 

 independently, capturing food and bringing 

 it to the mouth. Their number varies con- 

 siderably, increasing with the size and age 

 of the animal. 



The part of the body which is usually at- 

 tached to some object is known as the foot 

 or basal disk and is referred to as the aboral 

 (opposite the mouth) end. The foot secretes 

 a sticky substance, and not only anchors 

 the animal when at rest, but also ser\'es as a 

 locomotor organ. The foot may also secrete 

 a gas bubble enclosed by a film of mucus. 

 This bubble raises the animal to the surface, 

 where it spreads out like a raft, the h}dra 

 hanging from the underside. In the com- 

 mon brown species Hydra oligactis,*" the 

 aboral region is a stalk, and the distal region 

 constitutes a sort of stomach; these two re- 

 gions together are known as the body col- 

 umn. A conical elevation, the hypostome, 

 occupies the oral (mouth) end of the body. 

 The hypostome is surrounded by tentacles 

 already mentioned and has an opening at 

 the top, the mouth. WTien the mouth is 

 contracted, as during rest or digestion, it is 

 a minute circular pore, but when swallowing 

 objects, it and the surrounding hypostome 

 can dilate to a rclativclv enormous diameter. 



Frequently specimens of the hydra are 

 found which possess buds in various stages 

 of development (Fig. 50). This is a form of 

 asexual reproduction, characterized by the 

 fact that many parent cells go to make up 

 the new individual, which is in contrast to 

 sexual reproduction, in which the new indi- 

 vidual arises from a single cell, the fertilized 

 egg. Sexual reproduction also occurs in the 

 hydra. Reproductive organs or gonads (Fig. 

 50) may be observed on specimens of the 



* Genus Pelmatohydra discarded, not sufficiently 

 distinct. 



