MOLLUSC-LIKE ANIMALS 73 



MOLLUSC-LIKE ANIMALS. 



Two very problematical groups of animals are frequently united under 

 the name Molluscoida, which means mollusc-like forms. The two groups are 

 not nearly related to each other ; indeed, many naturalists to-day deny that 

 they have any connection, but until the question is settled it is convenient 

 in a popular work to consider them together, especially since there is very 

 little of general interest to say concerning them. In their general appear- 

 ance thev do not in the least resemble each other ; one looks to the casual 

 ol -server much like a bivalve mollusc, while the other bears a strong super- 

 ficial resemblance to a coral. The members of the first group are called 

 Brachiopods (arm-footed), the second Polyzoa or Bryozoa (many animals 

 or moss-animals respectively). The brachiopods are all marine, but a 

 large number of the moss-animals are found in fresh water. 



The history of the various positions assigned to these two groups is 

 worth mentioning, as it affords an excellent example of the way in which 

 each new discovery changes our previous conceptions of nature. The posi- 

 tion of neither is, as we have just said, still unsettled, but how long it will 

 remain so is a question. For many years there existed not the slightest 

 doubt but that the moss-animals should be ranked with the corals. They 

 formed a calcareous skeleton containing numerous little pits, in each of 

 which an animal was seated. This animal further resembled a coral polyp 

 in that it had a circle of tentacles surrounding the mouth. 



When a true study of these forms was instituted, it was soon realized 

 that they had not the slightest affinity with the corals, but that in every 

 point of structure they differed from them. The attempt was then made 

 to find some other group with which to place them. In those days 

 naturalists recognized only four great divisions of the animal kingdom ; 

 indeed, within very recent times it was considered the rankest heresy to 

 suggest that there might, by any possibility, be more. Therefore if it 

 were shown that the moss-animals were not radiates, they must belong 

 to one of the other three great groups. It was readily seen that they 

 were not vertebrates, and the question of their being articulates was 

 settled as readily, and also in the negative. Therefore they could only 

 be molluscs. 



