1891.] Mr Cooke, On Parasitic Mollusoa. 219 



very similar modification occurs in the radula of Sistrum spectrum 

 Reeve, a species which is known to live parasitically on one of 

 the branching corals. Here the laterals differ from those of the 

 typical Purpuridae in being very long and curved at the ex- 

 tremity. The general effect of these modifications appears to 

 be the production of a radula rather of the type of the vegetable- 

 feeding Trochidae, which may perhaps be regarded as a link in 

 the chain of gradually degraded forms which eventually terminate 

 in the absence of the organ altogether. The softer the food, the 

 less necessity there is for strong teeth to tear it ; the teeth either 

 become smaller and more numerous, or else longer and more 

 slender, and eventually pass away altogether. It is curious, how- 

 ever, that the same modified form of radula should appear in 

 species of Ovula (e.g. ovum), and that the same absence of radula 

 should occur in species of Eidima (e.g. polita) known not to be 

 parasitical. This fact perhaps points back to a time when the 

 ancestral forms of each group were parasitical and whose radulae 

 were modified or wanting, the modification or absence of that 

 organ being continued in some of their non-parasitical descendants. 



(2) Exhibition of models of double supernumerary appendages 

 in Insects : also of a mechanical method of demonstrating the 

 system upon which the Symmetry of such appendages is usually 

 arranged. By W. Bateson, M.A., St John's College. 



(3) On the nature of the excretory processes in Marine Polyzoa. 

 By S. F. Harmer, M.A., King's College. 



[Abstract: reprinted from the Cambridge University Reporter, May 26, 1891.] 



This communication was the result of an occupation of a 

 University table at the Zoological Station at Naples during the 

 Easter Vacation of 1891. 



Observations were made on the manner in which various 

 artificial pigments were excreted in Bugula and in Flustra, on 

 the lines adopted by Kowalevsky (Biolog. Gentralblatt, ix., 1889 — 

 1890, pp. 33 etc.) for other Invertebrates. The general result of 

 the experiments was to show that excretion is not performed by 

 organs comparable with nephridia, but that this process is carried 

 on by free mesoderm cells, and to some extent by the connective 

 tissue and by the walls of the alimentary canal. Evidence was 

 obtained to show that the periodic loss of the alimentary canals 

 leading to the formation of the " brown bodies " may be regarded 

 as, to some extent, an excretory process. 



