32 



ZOOLOGY. 



on the other liand, itself brings forth a progeny, which returns in its form 

 and nature to the ])arent animal, so that the maternal animal does not meet 

 with its resemblance in its own brood, but in its descendants of the second, 

 third, or fourth degree or generation ; and this always takes place in the 

 different animals m' liich exhibit the phenomenon in a determinate generation, 

 or with the intervention of a determinate number of generations. This 

 remarkable precedence of one or more generations, whose function it is, as 

 it were, to prepare the way for the later succeeding generation of animals, 

 destined to attain a higher degree of perfection, and which are developed 

 into the form of the mother, and propagate the species by means of ova, 

 can, I believe, be demonstrated in not a few instances in the animal 

 kingdom." 



3 4 5 6 



When a medusa, as Aurelia aurita {pi. 76, ßg 74), produces an eg;g, the 

 progeny resembles an animalcule (Diagram, ß-g. 1), which moves in the 

 direction of the arrow by means of vil)rilUv. The anterior extremity has a 

 round sucker (but not a mouth), by means of which, after several changes 

 of form, it attaches itself to some extraneous object (Diagram, ^^. 2). The 

 changes still continue, the two projections at the free end are extended, a 

 mouth is formed in the centre, and a second pair of projections arises 

 between the first. About the fifth or sixth day the four tentacles have become 

 longer, and the body quadrate, and the animal now constitutes the supposed 

 perfect genus, Scf/pMstonia (Diagram, ^5,?. 3), of Sars. In the next place, 

 four additional tentacles arise between the four earlier ones, and this 

 production continues until the number equals twenty-eight or thirty, and by 

 this time the animal resembles a polyp. 



In the subsequent changes an entirely new phenomenon is observable. 

 The free extremity of the body begins to show indications of a division into 

 segments, of which one is shown in Diagram, ßg. 4 ; the length and 

 number of segments increase, the body becomes cylindrical, and is now 

 about a line in diameter (its original size being that of a grain of sand), the 

 upper margin of the segments becomes free and divided into lobes 

 (Diagram, j?^. 5), capable of independent motion, when the form constitutes 

 the supposed genus, Strohila, of Sars, named from its resemblance to the 

 cones of a pine tree. 

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