KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 59. N:0 3. 51 



Even in a very young stage (shell 3 mm) the anterior coecal appendage was 

 observed on the left side of the stomach. There was also a large primary liver duct 

 on the left and a division of the stomach as in the adult, but no trace of a posterior 

 coecal appendage. 



In the oesophagus the epithelium shows near the mouth numerous small folds, 

 and farther backward one dorsal and one ventral furrow are seen, situated some- 

 what obliquely to the median line, but no regular plication like that of Ch. pellucida 

 can be established. A maximum of 8 or 9 small lists appear on each side of the 

 dorsal and the ventral furrows, which become lateral towards the mouth, while the 

 interjacent thick epithelium forms the lips. 



The circulatory system (fig. 57). The ascending trunk of the anterior aorta 

 passes in the median line above the stomach towards the front; it descends 

 again on the left side of the oesophagus. On the left of the mouth it gives off an 

 arteria pedalis and passes över to the anterior adductor. The descending trunk 

 of the anterior aorta is bipartite into two branches, the larger one plunges into 

 the left side of the body outside the intestine, approaches the stomach and encircles 

 it beneath the coecal appendage, while the smaller branch runs superficially on the 

 left side of the body. From the blood sinus there also emerges a short and narrow 

 branch to the right through the posterior parts of the liver and the genital coeca. 



The nervous system (fig. 58). Buccal ganglia exist, separated from the cerebral 

 centra. The cerebro-visceral connectives pass, on the left side, in the front part 

 immediately within the genital coeca, only slightly embedded in the liver, and nearer 

 to the outer surface of the body than to the stomach; on the right side somewhat 

 deeper. A median commissure exists in front of the pericardium. In the young 

 stage the position of the cerebro-visceral cords is the same. 



The gills. In the young stage examined (fig. 59) the anterior gill was just 

 beginning to be folded, while the posterior one still had a smooth surface. A still 

 younger specimen showed just the first step of development of the back gill, re- 

 presented by a series of small curved filaments appearing as projections from the 

 ends of those of the anterior gill. 



In the adult stage (fig. 60) each fold of the gills contains about 22 filaments. 

 These are connected by dense interfilamentar vessels. Folds of the same lamella are 

 connected by interplicatural bridges between the adjacent principal filaments; within 

 one single fold there also occur connections between opposite filaments which may 

 be named intraplicatural. The intruding folds of the opposite lamellae are combined 

 interfoliarly. Along the principal filaments there run vessels in the same alternation 

 as in Echinochama, that is one arterial alternates with a venous in the next fold. 

 The septa joining the venous vessels begin to appear nearer to the gill axis than 

 those of the arteriae, which are not met with until deeper in the gill. Every fifth 

 venous septum is higher than the surrounding ones. All the venous vessels run 

 from the marginal vein of the reflected lamellae down along the principal filaments, 

 turn up into the opposite lamella and continue towards the axis of the gill; they 

 do not, however, reach the branchial vein contained into it, but are obliterated be- 



