74 



FROF. AGASSIZ S 



those germs undergo in various families of Worms 

 seem to differ \\idely ; and indeed, among Worms 

 every where, there are types which are so widely 

 different in their outlines as scarcely to afford char- 

 acters by which to combine them. 



[PLATE XXVII- YOUNG WORMS J 



Is will be a great difficulty to find Anatomical 

 as well as Zoo'ogical terms to constitute in?o one 

 class all these various forms, (Plates XXVIII, 

 XXIX and XXX) an<5 those which are represent- 

 ed there, (Plates XXXI and XXXII ) Neverthe- 

 less, in tracing the> intermediate forms, we are 

 compelled to bring them into one and the same 

 group. 



[PLATE XXVIIT -WORMS WITH COLORED BLOOD] 



The class of worms, as I circumscribe it here, 

 contains numerous and very diversified types, as 

 well hy their internal .structure, as by their exter- 

 nal form: so that it is difficult to assign to all of 

 them common characters. The Intestinal Worms, 

 formerly considered as a class hy themselves, can- 

 not be separated from the true Annulata. There 

 ;ire intermediate forms between the two groups 

 For instance the Trematoda, which are closely al- 

 lied to Planaria, the AscariX which resembles Lum- 

 bricus, imd so on. The Intestinal Worms, gener- 

 ally speaking, have their body naked 5 the Acan- 

 thocephala only have hooks of fringe-like appen- 

 dages. Among Annulata there are, however, 

 types which cannot be compared with any of the 

 Intestinal Worms; as the Tubulibranchiata and 

 Dorsibranchiata. Among these there are some in 

 which the lateral appendages of the body are uni* 



[PLATE XXIX VARIOUS WORMS.] 



[I'L\Tf XXX E RTH \V'l)-RM AND I J T- A \' ); I A . 



form tor its wuule length; mothers, ttie appen- 

 dages of the anterior, middle and posterior region 

 of the body differ among themselves, and assume 

 even an entirely different character. In some, the 

 rings are generally provided only with a few stiff 

 hairs, whilst the head is surrounded with tufts of 

 respiratory fringes, and other appendages, in va* 

 rious degrees of development. Nevertheless, 

 through all that diversity, there is a common type 

 which can be easier understood than properly de- 

 scribed or defined. 



The development of the class of Worms varies 

 according to its types. In some, the yolk sub- 

 stance, after having been indefinitely subdivided 

 into homogenous little spheres or cells, assumes a 

 rotatory movement, sustained by vibrating Cilia, 

 which have been formed upon its whole sur- 

 face. Such are the Planarioe, &c., &c., whose 



