378 



THE ANIMAL KINGDOM 



Cilialedtracl-\ fA 



C oelom,' 



-C oel. pore- ^^%r^ J 

 Gill slits 



Coe-loTTi'^ 

 Coe-lom — = '~^' 



illL^Ciliatedtracl^ |^| 



Coelom, 



Coe-1 . pore 

 Coelom 



oe 



-Proboscis^ 

 Stomochorci' 

 Colleur 



Ciliated rirzd — 



'reoral lobe. 



lorn. 



' i^^iVi,**' 



Moalh 

 Co elom. 



H 



Figure 19.18. Diagrammatic side views of larvae of the Hemichordata (A to D) and 

 of the Echiiiodermata (£ to H). Although the early stages {A and E) are similar, after 

 different patterns of metamorphosis (B, C, and F, G), the end results are strikingly 

 different (D and H). The ciliated tract has been omitted from G, where it is somewhat 

 more lobulated than in f . ft has disappeared in C, D and H. 



The close relationship of these phyla is suggested by their develop- 

 ment. Not only are the early stages oi some hemichordates and some 

 echinoderms very similar, but the development of the hydraulic mech- 

 anisms shows that they are essentially homologous. 



In both phyla the eggs usually divide in simple fashion into 2, 4, 8, 

 etc., cells with no evidence of a specialized pattern such as spiral cleav- 

 age. Gastrulation (Fig. 19.17) is accomplished by simple invagination, fol- 

 lowed by a concentric ingrowth of the blastopore rim. In both phyla the 

 blastopore becomes the anus, and the mouth forms as a new opening some 

 distance away. On this account these phyla are called deuterostomous 

 ("second mouth"). 



In both phyla the coelom usually forms as pouches from the archen- 

 teron of the gastrula. The coelom (Fig. 19.17) typically has three por- 

 tions, one which is usually unpaired and two that are paired. The un- 

 paired portion (coelomj) forms as a pouch from the anterior end of the 

 archenteron. The paired portions (coelom^ and coelom^) may form as 

 posterior growths from coelom^ or as separate pouches from the sides of 

 the archenteron. Other variations also occur. Since several variations are 

 found in both phyla it is concluded that the final result (three portions) 

 is of more significance in a study of relationship than is the particular 

 way in which they are formed. 



