110 The Microscope. 



Observation has shown that in several of the Actinozoa 

 the different regions of the mouth have different functions. In 

 shape, the mouth is more or less oval, or even slit-like. At one 

 end a current of water is drawn in, bringing with it oxygen and 

 food, while at the other end the current flows in the opposite 

 direction. We have only to conceive the " mouth " closing in 

 the middle to get a veritable alimentary canal, (with mouth 

 and anus) traversing the length of the body. It has recently 

 been shown that in several forms (Peripatus, Serpula,etc.,) the 

 blastopore does thus close in the middle, the ends remaining 

 open and forming the mouth and anus of the adult. Further, 

 the digestive pouches of the sea-anemone need only to become 

 separated from the central cavity to become comparable to the 

 ccelom, and the walls to the mesoblast, of the higher animals. 



With the closure of the mouth in the middle, the sides of 

 the nervous ring would be drawn closer together, and then the 

 animal might be described in the following manner: It has 

 an elongate body with a mouth near one end and an anus near 

 the other, the alimentary canal traversing the distance between 

 them. On either side are a series of closed sacs arranged one 

 after another ; while the nervous system consists of two cords 

 parallel with each other and with the intestine, and connected 

 with each other in front of the mouth and behind the intestine. 

 If now transverse connections arise from one nervous cord to the 

 other between the mouth and the anus, corresponding in num- 

 ber and position to the ccelomatic pouches, the definition, so far 

 as it goes, will apply perfectly to an insect, a worm, or a crus- 

 tacean ; with one exception, — the nervous loop behind the anus, 

 and even this persists in some forms. Taking away the nervous 

 loop in front of the mouth, as well as that behind the intestine, 

 and the description applies to a vertebrate. Thus as will read- 

 ily be seen, our sea-anemone is converted into a segmented 

 animal. We have derived from it a segmentation of the body 

 cavity, a mouth and anus, and a longitudinal nervous cord. 

 Other metameric structures can be derived from it. 



It will be remembered that the pouches of the sea-anemone 

 were connected with each other and with the exterior. These, 

 external openings, among other functions, have that of afford- 

 ing a passage of waste products from the body. If we now con- 



