2 Memoir Sears Foundation for Marine Research 



for the first two) are dorsal and ventral in origin, but the dorsal and ventral roots do not 

 join, and there are no ganglia on the dorsal roots. The muscular system is segmented, the 

 successive muscle blocks, or myotomes, being separated one from the next by septa of 

 connective tissue, or myocomma. The final number of myotomes is established early in 

 life, but the number is somewhat variable in every species. The gonads are segmented. The 

 circulatory system is very simple; there is no heart, but the larger blood vessels are peri- 

 staltically contractile. There is a well developed coelom, or body cavity. The outer surface 

 of the body is clothed with an epidermis consisting of a single layer of columnar epithelial 

 cells, without scales or other hard epidermal structures, and without cilia except in the 

 mouth, pharynx, atrial cavity and intestine. There are no eyes and no limbs. The sexes are 

 usually separate although similar in external appearance, but hermaphrodites have been 

 reported on several occasions. Development is described below. 



The Lancelets differ from all the higher groups of fish-like animals — cyclostomes, 

 elasmobranchs, chimaeroids, and bony fishes — in the following important morphological 

 features. 



A. Their epidermis consists of a single layer of cells of ectodermal origin in con- 

 trast to several layers of cells in all higher groups. 



B. They have no hard epidermal or tooth-like structures of any sort. 



C. They have no eyes, no external nostrils and no true ears. 



D. When adult, the pharyngeal region with the gill clefts is enclosed, on the ventral 

 side, in a so-called atrial cavity. 



E. The gill clefts increase in number throughout life whereas in all the higher 

 groups their number is fixed. 



F. They have no specialized internal respiratory structures, no true brain, no heart, 

 no trace of a cranium and no hard vertebral structures, cartilaginous or bony. 



G. The notochord extends forward beyond the anterior end of the dorsal nerve tube. 

 H. Their blood is colorless, without red corpuscles. 



I. The neural canal, entirely closed dorsally in higher vertebrates, extends through 

 the dorsal wall of the nerve tube as a longitudinal fissure, reminiscent of the ectodermal 

 infolding by which the tube is formed. 



J. The excretory organs are nephridia-like rather than kidney-like, consisting of 

 numerous (up to 91) pairs of tubules in the pharyngeal region, each discharging inde- 

 pendently into the atrial cavity. 



K. The gonads are numerous, compared to only a single pair in higher groups, and 

 segmentally arranged; each discharges its products directly into the atrial cavity, there 

 being no permanent genital ducts. 



L. The lining of the intestine bears cilia. 



The relationship that the Lancelets bear to the Cyclostomes and to higher fishes has 

 been actively discussed, one view being that they represent the specialization of some 

 primitive prevertebrate stage in evolution, another that they are degenerate descendants 

 of some early type of vertebrate comparable to the Cyclostomes that have developed pe- 



