LECTURES ON MOLLUSCA. 279 



chains of compound animals. That they were the same, was first dis- 

 covered by Chamisso, the author of the well-known " Man without a 

 Shadow." The solitary Salp always gives birth to the compound, 

 and those again to the single. Doliolum is* intermediate between 

 Salpa and Pyrosoma. 



Family Appendicular:: ad^;. (Larval Ascidians.) 



The minute Appendicularias appear as cloudy patches of red color- 

 ing matter in the northern seas. They are little tadpole-shaped crea- 

 tures, and resemble the larval stage of the higher tribes of Tunicaries, 

 arrested at the first period of growth. 



CLASS POLYZOA. 



Among the creatures generally grouped together as zoophytes, and 

 forming the structures usually known as "Corallines," "Sea-weeds," 

 &c.j are many which are found to have a much more complex organi- 

 zation than the rest. There is an excurrent opening distinct from the 

 inhalent cavity ; and though their general habit of life resembles the 

 true zoophytes, yet there is sufficient analogy between them and the 

 compound Tunicaries to entitle them to a place in the molluscarf sub- 

 kingdom. They differ from even the lowest Tunicaries, in not having 

 any special circulating vessels; the fluids being generally transmitted 

 through the transparent mass of the tiny bodies. They have been 

 designated both as Polyzoa and Bryozoa; the former name being the 

 earliest, the latter the most distinctive as a class. By some authors 

 they are considered as superior Radiates, by others as degraded Mol- 

 lusks. The balance of characters seems in favor of the latter view; 

 but as they are more conveniently studied in common with the true 

 zoophytes, and are generally described in treatises concerning the 

 latter, they will not be further considered here. Those who are at 

 the sea-shore, and can examine the " sea-mats" and Lepralias in their 

 living state under the microscope, will do well to examine the differ- 

 ences between them and the common Sertularian Polypes. Some of 

 the forms are peculiar to fresh waters. The test formed by their com- 

 pound zooids is often somewhat calcareous. Their remains are ex- 

 tremely abundant in the Coralline Crag ; and even in the paleozoic 

 rocks, they play an important part among the fossil keys to knowl- 

 edge. It must be borne in mind however that many of the objects 

 described loosely as Bryozoa have no relation to this class. 



Those who desire information on this interesting class of creatures 

 are referred to "Johnstone's British Zoophytes," and to the works of 

 George Busk, Esq., published by the British Museum. 



On bringing to a close this brief digest of our existing knowledge of 

 molluscous animals, any one who will take the trouble to compare the 

 nomenclature and arrangement here adopted with that of any one or 

 more of the principal treatises on the subject, will be struck with the 

 general want of harmony which prevails among the different authori- 

 ties. It will not help us out of our difficulties to ignore their exist- 



