ON THE ANATOMY OF AMPHIOXUS LANCEOLATUS. 391 



which it lias neither jaws to comminute, nor powers of 

 stomach to digest. 



The branchial ribs I do not consider as parts of the neuro- 

 skeleton, as they bifurcate to enclose the heart, this organ in 

 the Lancelet being contained in a sac resembling the cartila- 

 ginous pericardium of some other fishes. They are repetitions 

 of' the hyoid bone developed for a new form of branchial 

 apparatus. They are true splanchno-ribs, parts of a splanchno- 

 skeleton, and analogous to the cartilages of the trachea and 

 branchial tubes (also repetitions of the hyoid bone) of the 

 higher vertebrata. Some of these splanchno-ribs, had branchial 

 clefts been developed, would have become true branchial 

 arches ; but just as in the vertebrata above the fishes, in which 

 the branchial clefts have disappeared, and tracheal cartilages 

 have become developed, so in this animal, in which the 

 branchial clefts have never appeared, cartilaginous arches 

 have become necessary for its peculiar aquatic respiration. 



The hyoid filaments of the Lancelet must not be con- 

 sidered as the analogues of the branchiostegous rays, which 

 spring from the peripheral aspect of the bone, but as developed 

 forms of the teeth or tubercles which are ranged along the 

 central aspect of the branchial apparatus of the higher fishes, 

 and which are occasionally highly developed for similar 

 purposes. As the upper jaw is developed from a cranium, 

 and the lower jaw is formed at a period posterior to the 

 appearance of the hyoid bone — the absence of these two bones 

 is a necessary consequence of the inferior position of the 

 Lancelet in the series of vertebrate forms. 



The plan of the circulation is simple, and in accordance 

 with the primitive condition of the respiratory apparatus, both 

 functions being performed in a manner closely resembling 

 that observed in certain annulose animals. The dorsal vessel 

 corresponding to the heart or branchial artery, and the 

 abdominal vessel to the aorta of the Lancelet, the lateral 

 communicating vessels of certain of the rings in the annelide 



