6os 



CHAPTER XXXIII. 



MOLL USCOLDEA. 



POLYZOA. 



We may consider here, under the name of Molluscoidea, the two 

 groups of animals which are known respectively as the Polyzoa and 

 the Brachiopoda. These two groups, in many respects closely 

 allied to one another, present affinities on the one hand to the 

 Worms, and on the other hand to the Mollusca, with both of which 

 they have been arranged by different systematists. In the present 

 state of our knowledge, however, it seems best to consider these 

 two groups separately, without referring them definitely to either of 

 the two sub-kingdoms above mentioned. The Tunicates, which 

 have also often been included amongst the Molluscoids, may like- 

 wise be in the meanwhile regarded as a separate division, which 

 finds its most natural position between the Mollusca and the Ver- 

 tebrata. It is not necessary, however, to further consider the 

 Tunicates in this work, since the palaeontological history of this 

 group of animals is an almost absolute blank. No fossil remains 

 of Tunicates have, in fact, been hitherto discovered except the 

 minute spicules of a species of Leptoclinum in the Pliocene beds of 

 St Erth. Many existing Tunicates, however, both simple and com- 

 pound, are known to possess more or less numerous calcareous 

 spicula in their tissues, and by the help of these it may ultimately 

 be found possible to trace back the existence of these singular or- 

 ganisms to an earlier period of the earth's history. 



The Molluscoidea may be briefly defined as unsegmented, simple 

 or compound animals, with bilateral symmetry. The mouth is 

 furnished with a crown of ciliated tentacles, or with spirally-rolled 

 ciliated processes. The nervous system consists of a single gan- 

 glion, or of an oesophageal nerve-ring with more than one ganglion. 

 A heart is absent or present. Under this head may be placed the 

 two classes of the Polyzoa and the Brachiopoda. 



