25 



Order PHARYNGOBRANCHII, Mull. 



The body is elongated and compressed, and there is no distinct head ; 

 the heart is also absent. The branchiae are free, pharyngeal and inoper- 

 culate. 



This group, embracing a single genus, the Branchiostoma of Costa, or 

 Ampkioxus of Yarrell, has been made by Miiller the representative of a 

 distinct subclass. Until its embryology is known, we may retain it among 

 the Dermopteri. 



A species of this genus was discovered by Dr. Stimpson and the author 

 at Beaufort, North Carolina, in the spring of the present year. 



For our knowledge of the anatomical characters of the preceding 

 groups, we have been chiefly indebted to the labors of the learned Miiller. 

 and the great English anatomist, Owen.* The classification here adopted 

 resembles theirs more than any others. The classification of Professor 

 Owen differs from that of Miiller in not recoguizing the existence of 

 subclasses, and in the less relative value assigned to the Pharyngobranchii 

 and Marsipobranchii. The present arrangement is, therefore, quite dif- 

 ferent from his, as we cannot consider the differences existing between 

 the Physostomi, I'liaryngognathi, Anacanthini and Acanthopteri as even 

 approximating in value to those between the orders just named, and the 

 Ganoids or Elasmobrauchiates. 



The relative standing and importance of the different subclasses of 

 Fishes above recognized have been variously interpreted by naturalists. 

 Some have placed the Elasmobranchii and Dermopteri near to each other, 

 and by Professor Agassiz they were formerly united in one order, for 

 which the name of Placoids was proposed. In the last classification of 

 Professor Agassiz, the Elasmobranchial order called Selachians and the 

 Marsipobranchial Dermopteri called Myzontes are regarded as belonging to 

 different classes, between which the Ganoids and Fishes, which are also 

 considered as classes, are interposed. Other naturalists have regarded 

 those two subclasses as representing the opposite extremes of the class, 

 and have placed the Elasmobranchii in the first rank and the Dermopteri 

 lowest. Those different allocations have resulted from different points 

 of view from which the groups have been examined. Those naturalists 

 who have approximated the Elasmobranchii and Dermopteri were princi- 

 pally influenced by the cartilaginous condition of the skeleton : those 

 who placed the Elasmobranchii in the highest rank looked to the special 



*We have been especially under obligation to the invaluable " Lectures on 

 the Comparative Anatomy and Physiology of the Vertebrate Animals, deliv- 

 ered at the Royal College of Surgeons of England in 1844 and 1846." The 

 first part, devoted to the Fishes, is rich in information on the structure of 

 those animals. It is the only one that has yet been published. 



