GLOSSOBALANUS MARGINATUS 585 



metry is apparent, and, as has been seen, it is not produced 

 altogether by distortion. 



The body is enclosed in a dorsal and ventral wall and on 

 each side is expanded into the genital wings, and the body- 

 wall at the origin of the wing on each side presents distinct 

 grooves— dorsal and ventral genital grooves. The ventral 

 groove has already been observed externally and extends as 

 has been noted, from the collar to the end of the genital reg/on 

 The dorsal genital groove likewise extends along not merelv 

 the branchial region but the whole of the genital region 

 The alimentary canal is resolved into a large dorsal pharynx 

 and a wide oesophagus. The walls of the pharynx are per- 

 forated by gills opening into gill-pockets which are provided 

 with openings in succession along a line medially situated to 

 the dorsal genital groove on the dorsal wall of the body. 

 The gills have the typical enteropneustan structure. The aili 

 septa or primary bars present ridges externally and are 

 supported by skeletal plates, which are conjoined and forked 

 internally and ventrally. Intervening between them are 

 the tongues or secondary bars, which are hkewise supported 

 by a pair of plates, but these are not fused. They lie on 

 either side of a narrow diverticulum of the body-cavity thus 

 not very far apart. They end below at the free end^of the 

 tongue. They are connected at intervals with the primarv 

 bars by synapticula and the shts are thereby converted into 

 a fenestrated succession of openings in each case (Text -fig 13) 

 I have not been able to find out exactly the number of synap- 

 ticula, but successive sections indicate the number to be 

 about ten. Nor can I state definitely the number of gills 

 but measurements made from the sections and from the 

 specimen show that the number is about forty to forty-five 

 on each side. 



The medial line is marked above and below by the dorsal 

 and ventral mesenteries with the dorsal and ventral nerves 

 and vessels. The circular layer of muscles is xexy thin 

 The longitudinal muscles reach their highest development on 

 the ventral wall of the body. They are still well developed 



