640 COMPARATIVE ANATOMY 



Sub-Phylum III. Hemichordata (Fig. 314). 



A rather doubtful form. There is a projection from the mid-dorsal 

 region of the digestive canal which looks somewhat similar to a noto- 

 chord. These animals have a collar and a proboscis. 



Order 1. Enteropneusta, which include worm-like forms such as 



Balanoglossus. 

 Order II. Pterobranchiata, sessile, tube-dwelling forms such as 



Cephalodiscus, and Rhabdopleura. 



Order III. Phoronidia, tubicolous forms such as Phoronis (Fig. 

 199). 



Sub-Phylum IV. Vertebrata (Craniata). 



1. The vertebrates show their segmentation in the adult form only 

 on the interior of the body, as for example, the metameric arrangement 

 of myotomes, sclerotomes, etc. 



2. A cuticular skeleton is absent, but there may be cornifications 

 of the epithelium, or ossifications in the dermal regions, such as the scales 

 of fishes, etc. 



3. An axial skeleton is present consisting of skull and vertebral 

 column. 



4. There are two kinds of appendages supported by the axial skele- 

 ton, namely, the unpaired fins (which occur only in fishes and Amphibia), 

 and the paired appendages (anterior and posterior), which are usually 

 present. 



5. The central nervous system is dorsal in position. The brain it- 

 self consists of five parts : the cerebrum, "twixt-brain," mid-brain, cere- 

 bellum, and medulla oblongata. 



6. Of the sensory organs, the eyes and ears are the most highly 

 developed. 



7. The respiratory organs arise from the entoderm of the pharynx. 

 Pharyngeal slits are present in the embryo. In terrestrial animals these 

 pharyngeal slits are later functionally replaced by lungs which develop 

 from the hinder portion of the pharynx. 



8. The heart lies ventrally in the pericardium. In gill-breathing 

 species it contains only venous blood, but in lung-breathing animals it is 

 divided into venous and arterial halves. The circulation is closed. 



9. The sexes are usually separate, while in most species the excre- 

 tory (nephridial) system forms the ducts for the reproductive (genital) 

 system. 



10. Reproduction is strictly sexual. 

 The classes of Vertebrata are as follows : 



Class I. Cyclostomata (Fig. 366). 



These are the round-mouthed eels without a lower jaw. Examples 

 are the lampreys and hagfishes. It is in this group that we find the only 

 vertebrate parasites. 



