568 MR H - D - s - COODSIR ON THE DEVELOPMENT, STRUCTURE, AND 



tozoa Diplozoon holds the same relation to Distoma. The Diplozoon has two 

 intestinal tubes, and two mouths, one for each body. The Distoma has two in- 

 testinal tubes, and only one mouth. In like manner also, the reproductive organs 

 are similar. It thus appears that the Distoma is only a Diplozoon folded on itself, 

 as Echinus is an Asterias folded back. There are certainly some few points of 

 material difference between these two animals, a circumstance we naturally look 

 for ; but these, if properly observed, must be traced to the difference of centraliza- 

 tion. Distoma is, therefore, superior to Diplozoon, as Echinus is to Asterias, 

 having a more centralized organization. 



The Acanthocephalous Entozoa of RUDOLPHI are the analogues of the Crus- 

 tacea. The Echinorhynci are typical of this group among the Entozoa. On com- 

 paring an Echinorynchus (PL XV., fig. 11) with a Crustacean, such as a Lernean 

 (PL XV., fig. 10), the relation between them is so like that of affinity, that they 

 were at one time grouped together in the same class. When the Lernean Crus- 

 taceans have passed their period of locomotive existence, and have become per- 

 manently fixed, their habits are exactly similar to those of the Echinorynchi, the 

 only difference being, that the former adheres to the external, and the latter to 

 the internal surface of the body of the infested animal. The Echinorynchi have 

 a number of short extremities or limbs near their head, analogous to similar organs, 

 or the atrophied limbs of the Lernese. There is this difference, however, between 

 these organs in the two sets of animals, namely, that in the one they have never 

 become developed at any period of life so as to suit the purposes of locomotion, 

 whereas in the other, and during its early stage of existence, they were fully de- 

 veloped and employed as organs of prehension and progression, but have only 

 become atrophied during the stationary or parasitic period of life. 



The next, and the highest forms of Entozoa, are the Coalelmintha, which, 

 on examination, will be found analogous to the Annelida. 



It is a remarkable circumstance, that looking on them collectively as classes 

 the Crustacea and Annelida are the first in the animal series possessing a 

 truly diseceous mode of generation. So is it with the analogues of these classes in 

 the Entozoa, viz. the Acanthocephala and Coalelmintha, the only groups in the class 

 which are truly bisexual. 



ANALOGIES. 



INFUSORIA. 



ENTOZOA. AJJALOGUES. 



I. Acephalocytis simplex I. Volvox glubator. 



POLYPIFJiKA. 



II. Diskostoma acephalocystis . . . II. Alcyonidium. 



(III. Tsenia III. Nemertes ?) 



