570 COMPARATIVE ANATOMY chap. 



interruption of the longitudinal musculature. In Ptychodera, the branchial pores 

 lie mediad of this line. 



The gills are, as a rule, paired, but in species of Ptychodera, those belonging to 

 one side may be shifted in front of those of the other side by as much as half the 

 breadth of a gill. 



At tlie posterior end of the branchial region, even in adult animals, new gills are 

 continually being formed. 



In Ptychodera clavii/cra, each gill -pouch has a long ventrally directed diver- 

 ticulum. 



The cavity of the branchial tongue is lined with endothelium, and traversed iu 

 various directions by fibres, some of which, no doubt, are muscular. 



The efferent section of the gill-pouches is provided with a musculature, which 

 cannot here he described. The pores also may be provided with an encircling sphincter 

 musculature of their own. 



In Balanoglossus Kowalcmhii, the posterior edge of the collar is continued back- 

 ward as two outgrowths, which cover the most anterior branchial pores. These 

 outgrowths have been called the operoula, and the small space they enclose, the 

 atrium. 



For the blood vessels and the skeleton of the branchial intestine, see below, 

 pp. 584 and 580. 



D. The afferent intestine, which follows the branchial intestine, 

 runs through the posterior gill-less part of the branchio-genital region, 

 and at its posterior end passes over into the hepatic or stomach intestine. 

 In some forms the afferent intestine is distinguished by the fact that 

 it sends off dorsally to right and left short canals, which open outward 

 on the dorsal surface. These efferent canals are known as the unpaired 

 intestinal pores, because they are for the most part unpaired. 



Special. — These unpaired intestinal pores are found in Scki-ocardinin hrccsilietise, 

 Glatuliccps Hacksii, and Gl. talaboti. In Schi. hrasiliense the openings are irregular, 

 either paired or unpaired. Twenty-nine in all have been observed, thirteen on the 

 left and sixteen on the right side, and among them seven pairs. The afferent section 

 of the alimentary canal is, in this species, distinguished by a strong circular 

 musculature. In Gl. Hacksii, nine unpaired pores were observed in the young 

 animals examined, the most anterior being on the right, and the rest on the left. 

 In Gl. talaboti, all the pores in this region are unpaired ; in the animals examined 

 they are arranged in nine groups at irregular distances from one another. The 

 efferent canals of each group probably open into a common ampulla, which, on its 

 part, opens outward through a single aperture. 



E. The hepatic or stomachal region of the intestine is, in all 



Enteropneusta, distinguished by the fact that its epithelium is ciliated 

 and contains numerous globules of a secretion, usually green in colour. 

 This section of the intestine seems to have a musculature of its own 

 only in ScJiizocardium hrasiliense, in which it is developed as a fine 

 layer of longitudinal fibres. The hepatic intestine is no doubt the 

 part of the alimentary canal of the greatest importance for digestion ; 

 the network of vascular capillaries, which will be described later, is 

 specially strongly developed in its walls. 



The hepatic intestine appears as a specialised section of the 



