AMPHIOXUS CYCLOSTOMI. 153 



the ventral wall of the atrial cavity is thrown into a number of 

 peculiar longitudinal folds which have been interpreted as kidneys. 



Development. The eggs undergo a total segmentation. The cells 

 resulting from segmentation form a blastosphere, which by invagina- 

 tion is transformed into a ciliated gastrula larva (fig. 598, A, B, G) % 

 The mesoderm is developed from lateral folds of the entoderm, and 

 at once segments into somites ; and at the same time the medullary 

 canal, which communicates with the alimentary canal behind and 

 opens freely to the exterior in front (fig. 598, D), is formed from 

 the ectoderm. Soon after the notochord arises from the endoderm. 

 The changes, which take place in the larval life, are introduced by a 

 considerable elongation of the body. In the further development 

 the larva is remarkable for a striking asymmetry (of somites, mouth, 

 anterior gill-slit, anus, olfactory organ). The branchial apparatus, 

 which is at first free, is afterwards covered by a reduplication of the 

 skin (formation of the atrial or peribranchial cavity). 



The only genus of the Leptocardii is Amphioxus Yarrel (BrancTiiostoma Costa) 

 including a single species distributed on the sandy coasts of the North Sea, of 

 the Mediterranean, and of South America. A. lanceolatus Yarrel, Lancelet. 

 The forms described as A. Belclieri Gray, from the Indian Ocean, and A. elon- 

 gates Sundev. probably belong to the same species. 



Order 2. CYCLOSTOMI* (MARSIPOBRANCHI). 



Vermiform fishes without pectoral or pelvic fins ; with cartilaginous 

 skeleton and persistent notochord. There are six or seven pairs of 

 pouch-like gills. The olfactory fossa is unpaired, and the circular or 

 semicircular suctorial mouth is without jaws. 



FIG. 599. Myxine glutinosa (regne animai). 



The Cydostomi have a cylindrical vermiform shape (fig. 599), and 



* Joh. Miiller, " Vergleichende Anatomic der Myxinoiden." Berlin, 1835-45. 



Aug.Muller, " Ueber die Bntwickdung der Neunaugen." Mutter's Archiv., 1856. 



Max Schultze, "Die Entwickelungsgeschichte von P. Planeri." Haarlem, 1856. 



P. Langerhans, " Untersuchungen iiber Petromyzon Planeri." Freiburg, 1873. 



W. Miiller, " Ueber das Urogenitalsystem des Amphioxus und der Cyclosto- 

 men. Jen. natunviss. Zeitschr., Tom. IX., 1875. 



A. Schneider, " Beitrage zur vergleichenden Anatomic und Entwickelungs- 

 geschichte der Wirbelthiere." Berlin, 1879. 



Calberla, " Zur Entwickelung des Medullarrohrs und der Chorda dorsalis der 

 Teleostier und der Petromyzonten. " Morpliol. Jahrb., Tom. III., 1877. 



