462 The Protochordata 



or lampreys, the Elasmobranchii, or sharks, the Ostracophori 

 the Arthrodira, and the Teleostomi, or true fishes. The first 

 two groups, being very primitive and in no respect fish-like 

 in appearance, are sometimes grouped together as Proto- 

 chordata, the others with the higher Chordates constituting the 

 Vertebrata. 



Other Terms used in Classification. The Leptocardii are some- 

 times called Acraniata (without skull), as distinguished from 

 the higher groups, Craniota, in which the skull is developed 

 The Leptocardii, Cyclostomi, and Ostracophori are sometimes 

 called Agnatha (without jaws) in contradistinction to the Gnath- 

 ostomi(ja,w mouths), which include the sharks and true fishes 

 with the higher vertebrates. The sharks and Teleostomes 

 are sometimes brought together as Pisces, or fishes, as distin- 

 guished from other groups not true fishes. To the sharks and 

 true fishes the collective name of Lyrifera has been given, 

 these fishes having the harp-shaped shoulder-girdle, its parts 

 united below. The Ostracophores and Arthrodires agreeing 

 in the bony coat of mail, and both groups now extinct and 

 both of uncertain relationship, have been often united under 

 the name of Placoderms, and these and many other fishes 

 have been again erroneously confounded with the Ganoids. 

 Again, the Teleostomi have been frequently divided into three 

 classes Crossopterygii, Dipneusti or Dipnoi, and Actinopterygii. 

 The latter may be again divided into Ganoidei and Teleostei 

 and all sorts of ranks have been assigned to each of these 

 groups. For our purposes a division into eight classes is most 

 convenient, and lowest among these we may place the Entero- 

 pneusta. 



The Enteropneusta. Most simple, most worm-like, and per- 

 haps most primitive of all the Chordates is the group of 

 worm-shaped forms, forming the class of Enter pneusta. The 

 class of Enteropneusta, also called Adelochorda or Hemichordata, 

 as here recognized, consists of a group of small marine animals 

 allied to the genus Balanoglossus, or acorn-tongues (fid\a(vos t 

 acorn; y^Goo-crai, tongue). These are worm -like creatures with 

 fragile bodies buried in the sand or mud, or living under rocks 

 of the seashore and in shallow waters, where they lie coiled in a 

 spiral, with little or no motion. From the surface of the body 



