260 MR GOODSIR ON THE ANATOMY OF AMPHIOXUS LANCEOLATUS. 



orifice of the ascidice, acting as a sieve in preventing the entrance of foreign bodies, 

 or of food, which it has 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 inclose the heart, this organ in the Lancelet being contained in a sac 

 resembling the cartilaginous pericardium of some other fishes. They are repeti- 

 tions of the hyoid bone developed for a new form of branchial apparatus. They 

 are true splancho-ribs, parts of a splanchno-skeleton, and analogous to the car- 

 tilages 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 car- 

 tilages 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 considered as the analogues 

 of the branchiosteogous rays, which spring from the peripheral aspect of the bone, 

 but as developed forms of the teeth or tubercules which are ranged along the cen- 

 tral aspect of the branchial apparatus of the higher fishes, and which are occa- 

 sionally 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 appear- 

 ance 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 man- 

 ner closely resembling that observed in certain annulose animals. The dorsal ves- 

 sel 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 performing the respiratory function, like the vessels of the branchial 

 chamber already described. The development of cardiac septa and of a liver follow 

 closely, if they do not accompany, the branchial fissures. The absence of such 

 fissures in the Lancelet sufficiently explains this deficiency of parts usually con- 

 sidered essential to the vertebrated animal. 



For similar reasons, true renal and generative organs do not appear in this 

 animal. 



The double row of isolated generative organs are in the normal position of 

 their embryonic representatives, and not more advanced in organization than the 

 Wolfftan bodies at their first appearance. How the contents of these ovisacs or 

 sperinsacs are conveyed to the exterior, it is difficult to say. If the abdominal 

 opening described by Professor RETZIUS actually exists, it appears to me much 

 more probable, -that it is an opening, not into the branchial, but into the peritoneal 



