POLYZOA. 305 



Respiration is provided for in the Polyzoa by the cihated append- 

 ages which surround the mouth ; they are at once tentacula and 

 branchiae. The animal presents no other trace of organs of the 

 special senses. A small ganglion and a few threads constitute all of 

 the nervous system which can be traced ; neither heart nor blood- 

 vessels have been found. 



The egg, in the case of the Polyzoa, gives birth to a young animal 

 covered with cilia on its surface ; it swims about freely until it has 

 chosen a convenient place in which it can establish the new colony 

 which it is to originate. At first, the number of the colony is only 

 increased by budding, but in a short time the individual polyps 

 produce eggs. 



From these remarks it will be seen that the animals of the Polyzoa 

 are more complex in their form and functions than those of the 

 Coelenterata, and the further study of their anatomy confirms this 

 conclusion. In their case the digestive organs are no longer a 

 simple sac with a single orifice ; there is a mouth, a pharynx, a gullet, 

 a gizzard, a membranous stomach and intestines, with a special 

 opening for these latter. We have descriptions of some species in 

 which the gizzard seems to be provided with a certain number of 

 interior teeth, forming a wonderful pavement — a living mill for the 

 purpose of grinding the food before it enters into the second stomach. 

 The organisation of this small creature reveals to our eyes a wonderful 

 amount of combination — of admirable art immeasurably surpassing 

 all that the most perfect human industry and human genius could 

 accomplish. 



After this general view of the organisation of the group, we shall 

 proceed to introduce the reader to some of the more characteristic 

 species. 



Under the leaves of water-lilies {IS'ymphca)^ pond-weed [Potamo- 

 geloji), or upon floating fragments of submerged wood, are generally to 

 be found certain Polyzoa, animals described by Trembley under the 

 name oi plumed polyps. These will probably belong to the genus 

 Plumatella {Fig. 120). These little diaphanous creatures constitute 

 colonies which under the microscope resemble small branching 

 shrubs ; they consist of small slender tubes fastened one to the other, 

 and having from forty to sixty retractile tentacula, which expand like 

 the petals of a flower ; they are furnished with vibratile cilia, the move- 

 ments of which serve the purpose of leading food into the mouth. 



Another genus, which is found in ponds in France, and which is 

 also found in fresh water in Britain, is the Cristatella of Cuvier. 

 " Perfect specimens of C. muccdo occur from six lines to twenty-four 



u 



