On the Distribution of the Invertehrata. 391 



the genital papilla is situated at the under part of the first 

 segment ; it is covered by two small lamellae, which in this case 

 did not sustain the eggs, which were found to be attached to 

 the first pair of ambulatory legs. The animal seems to carry 

 them in a similar manner as the pycnogonid Nymphon. 



Development. — The eggs contained embryos having already the 

 antennae, the five pairs of legs, and the abdominal feet ; they show 

 that Thaumops has to undergo no metamorphosis, and that the 

 young ones leave the eggs with all their appendages well de- 

 veloped. 



Mode of life, — It could not be made out whether T. pelhmda 

 inhabits the deep sea, or whether it is, like Phronima, a pelagic 

 animal, having been caught by the trawl only as the latter came up 

 from the depths. 



II.M.S. ' Challenger,' Teneriffe, 

 February 13, 1873. 



March 20, 1873. — Mr. George Busk, Vice-President, in the Chair. 



"On the Distributioh of the Invertehrata in relation to the 

 Theory of Evolution." By John D. Macdonald, M.D., F.R.8., 

 Staff Surgeon R.N., Assistant Professor of Naval Hygiene, Netley 

 Medical School. 



All organized beings exhibit both structural and functional con- 

 ditions, forming the grounds of comparison by which natural 

 affinities in smaller groups, and points of difference in larger ones, 

 are detected and established in systematic classification. 



General anatomical or physiological considerations in agree- 

 ment are usually of more importance than the harmony of single 

 or special conditions of either description ; and though structural 

 characters, as a rule, are superior to those of a functional nature, 

 uuich may be learnt from an arrangement founded on physiological 

 principles alone. I have elsewhere pointed out the deceptiveness 

 of takiug the habit of life as a guide in classification, though this 

 is adopted by many zoologists ; for essentially different types may 

 Uve under precisely similar circumstances, or the habit of life 

 may be very different in the members of the same type. Thus, 

 if we look upon a pectinate gill for aquatic respiration, fluviatile or 

 marine, and the amphibious coincidence of this with a puluionary 

 chamber, or the presence of the latter cavity alone in purely 

 terrestrial Gasteropods, as groupmg characters, nothing can be 

 more erroneous ; for all these conditions of the respiratory system 

 are to be met with in unequivocal examples of the same group, 

 anatomically defined, as in the Nerite alliance, or that of liissoa for 

 example. Nevertheless animals so simple in their nature as the 

 Protozoa may be distributed physiologically, with some show of 

 truthfulness in the resulting scheme. 



Passing the leading tjq^es of the Protozoa in review, we notice 

 that the Gregariuidaj alone are essentially parasitic in their 

 habit of life, obtaining nutriment from materials elaborated by 



