THE MAESIPOBEANCHII. 121 



exposed on the surface of the body. But, before long, a 

 longitudinal fold is developed npon each side, and grows 

 over the branchial apertures. The two folds eventually 

 coalesce on the ventral side, leaving only the abdominal 

 pore open. One cannot but be struck with the resem- 

 blance of these folds to the processes of integument which 

 grow over the branchiee of the amphibian larva ; and, in 

 like manner, inclose a cavity, which communicates with the 

 exterior only by a single pore. 



In a great many of the characters which have been enu- 

 merated — as, for example, in the entire absence of a distinct 

 skuU and brain, of auditory organs, of kidneys, of a cham- 

 bered heart ; in the presence of a saccular liver, of ciliate(^ 

 branchiae and alimentary canal ; and in the extension of the 

 notochord forwards to the anterior end of the body — Amphi- 

 oxtis differs from every other vertebrated animal. Hence 

 Professor Haeckel has proposed to divide the Vertebrata 

 into two primary groiips — the Leptocardia, containing 

 Amphioxus; and the Pachycardia, comprising all other 

 Vertebrata. The great peculiarities in the development of 

 Ampliioxus, and the many analogies with invertebrate ani- 

 mals, particularly the Ascidians, which it presents, lend 

 much support to this proposition. 



No fossil form allied to Ampliioxus is known. 



IT. The Marsipobeanchii. — In this order of the class 

 Pisces the integument is devoid of scales or bony plates. 



The spinal column consists of a thick persistent notochord 

 enveloped in a sheath, but devoid of vertebral centra. The 

 neural arches and the ribs may be represented by carti- 

 lages, and there is a distinct skull presenting cartilage at 

 least in its base, and retaining many of the characters of 

 the fcetal cranium of the higher Vertebrata. _ The notochord 

 tenninates in a point in the base of this cartilaginous skull 

 behind the pituitary body ; and the skull is not moveable 

 upon the spinal cohimn. There are no jaws ; biit the palato- 

 pterygoid, the quadrate, the hyomandibular, and the hyoi- 

 dean apparatus of higher Vertebrata, are imperfectly repre- 



