8 CTENOPIIOR^. 



duced from eggs, and that at an early age tliey gave unmistakable 

 signs of their parentage. McCrady's observations showed us how great 

 were the changes of figure their young undergo before they assume 

 the aspect of the parent. It has been my good fortune to trace these 

 changes in several of our species of Ctenophoraa somewhat in detail, 

 and I shall make use of the material thus afforded in discussing the 

 position of these animals, as well as their pretended bilaterality, and, 

 by comparing their mode of development with that of PolyjDS, Acalephs, 

 and Echiaoderms, endeavor to ascertain whether their association with 

 them into one great branch of the Animal Kingdom is true to nature, 

 or whether the aflfiuities between the mode of execution in the plan of 

 the members of the Coelenterata are really of such a character as to 

 justify their separation from the other Radiates as one great branch 

 of the Animal Kingdom. 



Let us first examine the character of the Coelenterata and of the 

 Eadiata as they are understood. What is common to Polyps, Acalephs, 

 and Echinoderms is a vertical axis, or rather an axis through which 

 we can pass a plane at right angles, and in this plane draw two axes 

 at right angles to each other. These axes, of course, are not equally 

 prominent in Polyps, Acalephs, and Echinoderms ; taking, for instance, 

 the three axes as we find them in some of the Spatangoids, we have a 

 vertical axis, a coehac axis, and a diacoehac axis, the mouth and anus 

 bemg placed in such a position with reference to the coeUac axis as to 

 give us a right and left, an anterior and a posterior extremity. In the 

 Acalephs, it is only among the Ctenophorae that we can distinguish 

 between the coehac and diacoeliac ; but we have neither right nor left 

 — no anterior or posterior — side ; while in Polyps we can distinguish 

 their axes with greater exactness than in the Hydroids and Discophorae. 

 We are so accustomed to impose our notions of symmetry on every- 

 thing we meet, that it is difficult to divest ourselves of the idea that 

 every animal has not necessarily a right and a left side, an anterior and 

 a posterior extremity ; we start with the idea that such relations must 

 exist in all animals, however disguised, and under this impression we 

 try to reconcile plans which are totally distinct. If, however, we admit 

 the idea of different plans as the foundation of animal life, we must 

 give up all attempt to find some passage from one to the other. Ani- 

 mals the equation of which could be represented by that of a sphere, 

 or by that of two parallel planes, or of a series of cylinders, or of two 

 parallel cylinders, can never pass from one to the other ; the equation 

 of a sphere cannot be transformed into that of a plane, nor into a 

 cylinder ; the equations representing each of these figures include, it 

 is true, all the possible spheres or all the possible cylinders which may 

 be constructed by changing the values of the variables, but can never 

 be transformed one into the other. The infinite variety of forms, and 



