508 SUMMARY OF CURRENT RESEARCHES RELATING TO 



up owing to their activity, has no use in an organism which lives a 

 parasitic life ; and the physiological process is therefore completely 

 similar to that which obtains in fattened cattle. 



The earlier part of the paper is taken up by (1) an account of the 

 presence of these forms in certain Ascidians, the author only finding 

 them in Phallusia mentula, and P. mammillata, where they are far 

 from being the only guests ; (2) a description of their external form ; 

 and (3) a systematic account of the species, which are arranged under 

 the genus Doropygus, with as subgenera, Doropygus and Notopteropliorus. 

 Seven species appear to be known. 



Organization of Trilobites.* — The veteran H. Milne-Edwards, in 

 discussing the results of the researches of Mr. Walcott,"j" concludes 

 that the alliance, on which he long ago insisted, between the Trilo- 

 bites, Isopoda, and Phyllopoda, is strengthened rather than weakened 

 by these studies ; he cannot believe that they were representatives of 

 the Arachnidan type from which the Limuli appear to have been de- 

 rived, and he thinks that a group composed of Trilobites, Limuli, and 

 Eurypterina would be altogether artificial and inadmissible into a 

 natural zoological classification. 



It is pointed out that although there is, at first sight, a very con- 

 siderable resemblance between young Limuli and young Trilobites, 

 yet that the latter soon become provided with thoracic segments, and, 

 to cite characters of less importance, they tend to become ornamented 

 with those long spiniform prolongations, the presence of which is so 

 characteristic not only of Zoeae, but of many adult Macroura. 



If we examine the respiratory organs of the Trilobites, we find 

 them to differ much more from the Limuli than they do from the 

 Branchiopoda or the Hedriophthalmata. The principal differences 

 between the external structure of a Limulus and of a Phyllopod or an 

 Isopod are to be found in the relations of the mouth to the appendi- 

 cular system, and the mode of division of labour between the different 

 parts. In the Limuli we find two distinct groups: one forms a 

 masticatory, prehensile, and ambulatory system, at the centre of 

 which we find the mouth; the other, the respiratory apparatus, is 

 situated more posteriorly, and presents none of the characteristic 

 forms of any Arthropod walking limb ; no known existing animal has 

 a similar structure, and no one of the recently observed facts leads us 

 to see any close resemblance to them in the Trilobites. Prof. Milne- 

 Edwards has now no doubt as to the existence of a long series of post- 

 cephalic limbs in the Trilobites, and the characters of these appear to 

 him to present a certain resemblance to those of Apus ; it is possible 

 that they were almost altogether homomorphous and natatory rather 

 than ambulatory. It is pointed out that we have an erroneous idea of 

 the essential characters of the appendicular apparatus of the Phyllo- 

 poda, if we imagine that they are always entirely soft and membranous ; 

 in Apus the coxopodite and some of the succeeding joints of the 

 internal ramus are thick and firm, and we can imagine that under the 



* Ann. Sci. Nat. (Zool.) xii. (1881) Art. No. 3, 33 pp. (3 pis.), 

 f See this Journal, i. (1881) p. 736 



