30 Memoir Sears Foundation for Marine Research 



for ease of identifying any Cyclostome that may come to hand; for that reason we have, 

 selected characters which are most easily used. 



References. All citations listed among the references, with the few exceptions noted, 

 were consulted in the original ; for a list of co-operating libraries, see the general discus- 

 sion for the section on Sharks. 



Sources of Material. The collection of Cydostomes in the Museum of Comparative 

 Zoology has been the chief basis of our studies, but the collections at the United States 

 National Museum and the Academy of Natural Sciences at Philadaelphia have also been 

 drawn upon. 



Class AGN AT HA 

 Subclass CYCLOSTOMATA 



Fish or fish-like vertebrates,^ eel-like in form, the skeleton cartilaginous or fibrous, 

 without bone; no definitely developed jaws or bony teeth; at least the rudiments of a 

 cranium present in the form of a simple trough below the brain in some, but partially 

 roofed in others; notochord not constricted at all segmentally; vertebral column repre- 

 sented by a simple notochordal sheath, without vertebral centra, but with rudimentary 

 neural arches (not joined above) in some; no shoulder or pelvic girdles, no paired limbs 

 and no true ribs; 6 to 14 pairs of gill pouches opening either directly into the pharynx 

 internally or into a separate respiratory tube, which in turn opens into the mouth below 

 the gullet, and opening to the exterior either separately or by a single aperture on each 

 side; skin without scales; nostril single, either opening into the mouth or not; intestine 

 with internal longitudinal ridges, or with a slight spiral fold; ear with either i or 2 semi- 

 circular canals only; no sympathetic nervous system, or spleen; heart without conus arte- 

 riosus; no cloaca, the genital apertures being separate from anus. Development oviparous, 

 with or without a definite larval stage ; the sexes separate or not. 



The Cydostomes are generally considered the most primitive of true vertebrates; 

 structurally they are the simplest. They are easily distinguishable from all the higher 

 fishes by their peculiar jawless mouths, by the fact that there is only one nostril, and by 

 the very primitive cranium.^ 



Key to Orders 



I a. Snout with prominent barbels; no separate dorsal fin; eyes not visible externally; 



nasal opening at tip of snout; mouth not funnel- or disc-like. Myxinoidea, p. 31. 



lb. Snout without prominent barbels; one or more dorsal fins separate from caudal; eyes 



in adult well developed, and visible externally; nasal opening on upper side of head; 



mouth opens as a funnel or disc. Petromyzonida, p. 43. 



1. Opinions differ as to whether the Cydostomes are to be regarded as a class distinct from the true fishes, or as a 

 subclass of the latter. 



2. For detailed accounts of the anatomy of the Cydostomes, see especially Lonnberg, Favaro, Mozejko and Rauther 

 in Bronn's Klassen, 6, Abt. i, Buch i, 1905-1924: 16-39, pl- ''3~3^> ^1*° Pietschmann, in Kukenthal, Handb. 

 Zool., 6, 1st half, 1929— 1935: 2-5. 



