50 H. WHITEHEAD ON BRITISH FRESHWATER RHABDOCOELIDA 



Specimens of this interesting Rhabdocoel were taken in one of 

 the ponds in Richmond Park, on the occasion of the Club's visit 

 on April 13th, 1912. The following week the writer took 

 specimens from a pond near Chigwell Row, Essex. 



It was noticed that the animals had a number of eggs (in one 

 instance 49 were counted) in the spongy body tissue, and 

 individuals in this condition avoided the light. As far as could 

 be ascertained, no eggs were deposited by the living animals, but, 

 on death, the eggs were liberated on the decomposition of the 

 body of the parent. So far none of these eggs have hatched. 



Prof. Sekera (16) of Tabor, Bohemia, succeeded in keeping 



specimens alive for some time, and the following notes are taken 



from the account of his observations. Young specimens were 



taken in ponds in March, when ice was still floating on the 



water. The animals were colourless, but as soon as they 



approached maturity, and the sexual pore developed, it was 



noticed that a few algal cells (zoochlorellae) had entered the 



body cavity by this means. Streaks of green granules then 



began to spread from this region and extend beneath the cuticle 



over the whole body, until finally the animal became quite 



green. (T would remark, in parenthesis, that mature specimens 



show distinct lines or bands devoid of zoochlorellae.) Solid 



food in the form of diatoms, rotifers, etc., was ingested during 



this period. While rapid division of the algal cells was taking 



place, they formed spherical or ellipsoid clusters, each group 



being surrounded by a colourless membrane. The membrane 



finally disintegrated and the algal cells were dispersed in narrow 



irregular lines or bands. The mature zoochlorellae showed no 



signs of an enveloping membrane. The animals exhibited at 



this period a distinct tendency to crawl towards light (phototactic), 



but sank to the bottom of the vessel at night. During the third 



week eggs were formed in the body cavity. The worms at this 



stage began to avoid the light and spent the whole day at the 



bottom of the vessel or under vegetation. During the first week 



in May the animals died off rapidly, and with the decomposition 



of the body the eggs were liberated. The algal cells were set 



free and continued to live, and developed an investing membrane, 



then passed into a resting stage, probably awaiting an opportunity 



of invading the next generation of Dalyellia. 



Prof. Sekera thinks that the alga is of little or no value to the 



