BRITISH MOLLUSCA. 



ACEPHALA TUNICATA. 



In every class of organised beings there are creatures 

 so constituted as to link the group to which they belong 

 with some other, or even to render their true position a 

 matter of question. Especially at the extremities, at the 

 lowest and highest portions of considerable sections, do we 

 find such beings. In so great a subdivision of the animal 

 kingdom as the Mollusca, we must expect to meet with 

 anomalous or connecting creatures ; and Avere we to begin this 

 history of our native species, in which we propose to treat 

 of them in the order of their ascent in the animal series, 

 with such as seem to us to mark the commencement of their 

 type, we should have to extract a very considerable chapter 

 from our esteemed friend Dr. Johnston''s " History of the 

 British Zoophytes." For the curious beings called Bryozoa, 

 or Ascidian Polypes, present so many characters in common 

 with certain undoubted Mollusca, especially with the 

 Tunicata, and so few comparatively with true zoophytes, 

 that in a natural classification they could not with propriety 

 be separated from the former class. Even that which was 

 supposed essentially to distinguish them from true Mol- 

 lusca, the absence of ganglia in their nervous system, has 

 been shewn to be incorrect, since Van Beneden and Pro- 

 fessor Allman have demonstrated the presence of a distinct 

 nervous system with a ganglion in certain species of 

 Bryozoa. 



Still the Bryozoa may be regarded as a lowest order of 

 Mollusca, linking that great class with the Zoophyta, and 

 distinguished from the true tunicated mollusks by the 



VOL. I. B 



