Marvels of Pond-Life. 1 1 D 



evident differentiation and complication of parts. Tlicy 

 belonged to the Pohjzoa or Bnjozoa^ a very important 

 division of the moUusca. The Pohjzoa are chiefly 

 marine^ and the common " sea-mat/' often erroneously 

 treated as a sea-iveed, is a well-known form. A species 

 of another order often picked up on our coasts is the 

 Sertularia, or Sea-Fir, composed of delicate branching 

 stems of a horny-looking substance, which, under a 

 pocket-lens, is found to contain an immense number of 

 small cells inhabited bv Polvps. It is instructive to 

 compare the two and note how much more advanced in 

 ..structure is the Polyzoon than the polyp. 



Polyzoa were formerly associated with the polyps, to 

 which they bear a strong superficial resemblance ; but 

 they are of a much higher degree of organization, as 

 will be seen by comparing w^iat has been said in a 

 former chapter on the Hydra, with the description 

 which we now proceed to abridge from Dr. AUman's 

 splendid monograph on the fresh-water kinds. In 

 order to get a general conception of a Polyzoon, the 

 Professor tells us to imagine an alimentary canal, 

 consisting of oesophagus, stomach, and intestine, to be 

 furnished at its origin with long ciliated tentacles, and 

 to have a single nervous ganglion on one side of the 

 oesophagus. We must then conceive the intestine bent 

 back till its anal orifice comes near the mouth ; and 

 this curved digestive tube to be suspended in a bag 

 containing fluid, and having two 0})enings, one for the 



* Polyzoa means " many animals," in allusion to their habit of living 

 in association. Bryozoa, " moss-animals," from some formuig cells 

 having that appearance. 



