150 MANUAL OF ANIMAL BIOLOGY 



over the gill filaments, which are developed in or near the walls of 

 the gill slits, and then to the exterior. In the aquatic Chordates, 

 such, for example, as a Fish, this movement of the water is essential 

 inasmuch as the exchange of gases necessary in respiration takes 

 place as the water bathes the highly vascularized gill filaments. 

 Having noted the characteristic features of chordate structure 

 we may now proceed to a consideration of the animal types which 

 are usually included in this phylum. Four major divisions, or 

 subphyla, of Chordata are recognized, the first three of which are 

 interesting from the comparative standpoint, but otherwise unim- 

 portant. These are as follows : 



A. Hemichordata. A small group of marine animals, usually 

 worm-like in appearance and with a proboscis, but which may 

 show great variation. The best known representative of the 

 Hemichordata is Dolichoglossus which is fairly common. It lives 

 along the shore embedded in the sand or mud and secures its 

 food in much the same way as the Earthworm, that is, by digesting 

 the organic material from the debris which passes through the 

 tubular alimentary canal. The notochord is short, the central 

 nervous system not highly developed, but the paired lateral gill 

 slits are prominent and functional. Colonial, deep sea species are 

 known which reproduce by budding. 



B. Urochordata. This subphylum contains several rather 

 common marine organisms which are commonly known as the 

 Sea-squirts, due to their habit of ejecting a stream of water when 

 disturbed. The mature individual of this group shows a degen- 

 erate condition as compared with the larva. The latter for a time 

 during early development is a small, free-swimming, tailed form 

 with well-developed Chordate features, somewhat closely resem- 

 bling the larval, or tadpole, stage of the Frog in general body plan. 

 It very soon attaches itself anteriorly to some solid object in the 

 water and undergoes a marked structural regression, or meta- 

 morphosis, to form a sessile adult which lacks the tail, notochord, 

 and central nervous system, but possesses many gill slits and 

 associated organs which serve both for respiration and for the 

 capture of food in somewhat the same way as in the Clam. The 

 adult is enclosed by a peculiar covering, or tunic, which is the basis 

 of the term tunicate, often used to designate this subphylum. 

 It is stated that the tunic is largely composed of cellulose, and that 

 this is possibly the only example in the animal kingdom of this 



