rilYUM CIKth'DATA 10/ 



hnigitiulinal iimsclr in.iss into scumcnls cullcil myolotnts or 

 myotnei'cs. 



Lociito the opt'iwnn of llic oral hood, which is a larnc <»v:il 

 aperture on the ventral surface of the cephahc end of the Ixxiy. 

 It is fringed on each side with about 12 ciHated tentacles or cirri, 

 known as the oral cirri. The <uiiis is on the left side of the anal 

 fin just at the base of the caudal fin. The atrinporc is a inediatj 

 aperture on the ventral surface between the cephalic end of the 

 anal fin and the converg(>nce of the metapleural folds. The 

 respiratory current entering the mouth leaves the body through 

 this pore. 



Internal Structure. The Digestive and Res pi rotary Orfians. — 

 The alimentary canal is essentially a straight tube. The oral 

 hood and cirri have been mentioned above. The walls of the 

 oral hood delimit a space called the oral cavity. The anterior 

 part of the enteron is the pharynx, the word here being used to 

 mean not merely a throat but a division of the enteron which 

 serves the double purpose of conveying both the respiratory 

 medium and the food. It is a compressed tube, occupying ai)out 

 one-half the total length of the aUmentary canal. It is surrounded 

 by a space, the atrial cavity, which will be seen in its proper 

 relations in the study of a transection of this region. The 

 pharyngeal walls are perforated by numerous sUtlike apertures, 

 the gill slits, running obUquely ventrad and caudad. Water 

 taken in at the mouth passes through the gill slits into the atrial 

 cavity from which it escapes through the atriopore. The 

 epithelium lining the gill slits constitutes the respiratory tissue. 



Between the cavity of the oral hood and the beginning of the 

 pharynx there is a circular sheet of tissue which appears as a 

 straight line when viewed from the side of the animal. This 

 membrane is called the velum. It is perforated by the mouth 

 opening. 



Caudad of the last gill slit the alimentary canal narrows and 

 continues as the intestine which, a short distance caudad of the 

 pharynx, gives off from the ventral side a blind pouch. This 

 evagination is the liver the blind end of which extends cephalad 

 on the right side of the pharynx. 



The Skeleton.— The skeleton is of the endoskeletal type and 

 extreme in its simplicity. It consi.sts of a flexii)le, rodlike 

 structure above the alimentary canal and extending throughout 

 the entire length of the body. This is the notochord which serves 



