42 Hjalmae Théel, 



Now the blind end of the archenteron commences to undergo a 

 series of remarkable changes. It assumes at first a rounded expanded 

 appearance, PL III, fig. 4.5, after which it flattens, becomes depressed 

 against the rest of the archenteron, fig. 47, and presents itself as a disc, 

 ß.q. 48, which grows larger, while its peripheral part bends towards the 

 archenteron, fig. 49. Simultaneously it becomes gradually separated by 

 constriction fig. 44 c, so as to form a vaso-peritoneal vesicle, which takes 

 a dorsal position and at the same time divides into two vesicles, which 

 arrange themselves on the right and left sides of the oesophagus, fig. 

 45 c. It seems to me that the separation of the two vesicles is almost 

 completed at the moment of the constriction of the whole. The left ve- 

 sicle places itself in communication with the exterior by the water-pore, 

 this being accomplished at the end of this pluteus stage, PL V, fig. 84. 

 the second day after the fecundation and before the anterior ventral 

 arms of the larva are formed, the funnel-shaped oral invagination of 

 the concave ventral surface takes place, PL III., fig. 41, in order to 

 meet the archenteron immediately after its separation from the vesicles 

 above mentioned. 



In the stages of Pluteus, in which an age of six days has been 

 reached, the digestive tract is well developed and rather complicated, its 

 different regions being sharply distinguished. The entrance to the mouth 

 presents a wide, slightly depressed space, PL F, fig. 82 — 83 m, well 

 limited by high columnar cells and extending over a good deal of the 

 anterior ventral concave area of the larva. Posteriorly the mouth nar- 

 rows into the œsophagus, fig. 82 — 8-3 oe, which is a rather long thick- 

 walled tube, lined with a high ciliated columnar epithelium, round which 

 fine contractile fibres are to be observed, the oesophagus thus acquiring 

 the capability of powerful contraction. But the oesophagus is also able 

 to shorten, and in such a state it presents rather a spacious cavity. 

 Besides, the larva often repeats a powerful swallow, in the course of 

 which a part of the gullet invaginates into the stomach. 



The gullet communicates by a wide constricted opening, dorsal 

 in position, with the large globular stomach, fig. 82 — 83 st., which fills 

 up a good deal of the larval body, and is lined with not very high 

 ciliated cells each containing a large round nucleus. At the ventral side 

 of the stomach and posteriorly a round opening leads into an intestinal 

 sac, fig. 82 — 83 r, which is obviousl}^ thin-walled and opens externally 

 through the blastopore, or rather anus, which is situated more in the 



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