DEVELOPMENT OF MYTILUS EDULIS AND ALCYONIUM DIGITATUM. 559 



by a slow, fine air jet to keep the food in circulation, and this is still 

 kept up, the mussels slowly increasing in size. 



The largest individual in the jars at present measures 2'2 mm. long 

 X 1'5 mm. high. 



Note. — On August 7th, 1912, several swimming larvae, including one mussel that could 

 both swim and crawl, were placed in a Brettit with a little of the water in which they 

 were brought in. The jar was then filled with outside water, and the mussel, tliree 

 Anomia sp. larvae, and several gastropod larvae developed and grew in this jar. No food 

 has been added. The mussel is much larger than those wholly reared in the Laboratory, 

 and measures 4 mm. Iongx2'0 mm. high. The Anomia sp. measures 10'25 mm. across 

 the widest diameter of the shell. 



2. Alcyonium digitatum. 



Male and female colonies of Alcijonium digitatum were placed in a 

 tank on January 26th, 1912, and from January 27th to February 3rd, 

 eggs in various stages of segmentation were pipetted out of the tank 

 where they were floating, into Breffits containing Berkefeld or outside 

 water. Hence the early critical stages of maturation and fertilization 

 took place in the tank water. On January 28th many advanced 

 morula passed into a curious irregular stage, which in turn became 

 a round ciliate planula. This elongated gradually to an oval swimming 

 planula, and as development proceeded the shape became pear-like, 

 the larva swimming with the broad anterior pole forward, and simul- 

 taneously rotating on its axis. The characteristic reddish-brown 

 colour of the egg gradually became pale cream as the larvae absorbed 

 the yolk, and planarian-like contractile movements were observed 

 when they were irritated by light, pipettes, etc. 



Later they floated towards the base of the Breffits or near the 

 surface film, with the long axis vertical and the anterior pole upwards, 

 and on February 6th, some larvae had fixed on the glass at the surface 

 film. Shrinkage now took place along the long axis, so that the oval 

 larva became short and dome-shaped, and through the glass Breffits 

 the eight mesenteries were visible. Some larvae settled on glass rods, 

 glass cover-slips, and pieces of paraffin which were introduced into the 

 Breffits at this time. By invagination of the ectoderm at the free 

 pole of the larva, the mouth and storaodeum now arose, — fifth day of 

 fixation, and as the yolk was absorbed it became paler and more trans- 

 parent. Later eight simple tentacles appeared round the mouth, 

 so that the larva now resembled a small anemone. By February 17th 

 the tentacles were well developed and bore 2-3 lateral branches, and 

 they waved gracefully about in the water if undisturbed, but retracted 

 completely if shaken or disturbed. Forty days later the solitary polyp 

 had grown very considerably, but no lateral buds had arisen. The 



NEW SERIES. — VOL. IX. NO. 4. MARCH, 1913. 2 



