GENERATION. 



433 



in another manner. The most remarkable ex- 

 amples of this are met with in Polypi, Entozoa, 

 and Annelida. When the Hydra viridis is cut. 

 through either longitudinally or transversely, 

 each segment continues to live and grow, and 

 is gradually furnished with those parts of the 

 body of which it was deprived by the division. 

 Thus, when the polype is divided across the 

 body, the part with the head and tentacula is 

 gradually furnished with a body, while ten- 

 tacula grow on the elongated extremity of the 

 other part. When, again, the animal has been 

 divided in a longitudinal direction, and four 

 tentacula are left on each part, the opposite 

 edges of each segment turn round and unite 

 so as to complete the tube of the stomach, 

 and four additional tentacula are formed upon 

 each. 



Segments of the Tape-worm, Filaria, and 

 also of some other Entozoa, are capable of 

 living after separation and being converted in- 

 to independent animals. In the Leech and 

 Earth-worm, as well as some other Annelida, 

 the division of the body into two or more seg- 

 ments is not invariably followed by death, but 

 some or all of the portions continue alive, and, 

 acquiring the deficient organs, become con- 

 verted into more or less perfect animals. 



Gemmiparous generation. The second form 

 of non-sexual propagation that deserves our 

 attention is that in which the new individual 

 grows upon the parent as a bud or sprout, at 

 first exhibiting little appearance of the form or 

 structure of the perfect animal ; gradually as- 

 suming its form while still attached to the 

 parent stem ; and being afterwards separated to 

 enjoy independent existence. 



The best known examples of this kind of 

 generation occur in the polypine and coralline 

 animals, and the process has been observed 

 with great attention by Trembley in the Hydra 

 viridis.* In this animal the young polype 

 makes its first appearance as a small conical 

 eminence on the body of the parent : this gra- 

 dually enlarges and becomes cylindrical ; a 

 cavity is formed in its interior, which at first is 

 separate, but afterwards comes to communicate 

 with the stomach of the parent, so that aliments 

 taken by the parent penetrate into the stomach 

 of the offspring. As the new polype enlarges, 

 the internal cavity opens at the free extremity, 

 where a mouth, provided with tentacula, is 

 formed. The young animal then catches and 

 swallows food for itself : this food at first finds 

 its way into the stomach of the parent, but after 

 some time all communication between the two 

 stomachs is prevented by the closure of the root 

 of the stem of the small polype ; and afterwards 

 the offspring is detached from the parent, be- 

 comes a separate individual, and in its turn 

 propagates new ones from its sides. The time 

 at which the separation takes place seems to 

 depend in some measure on the quantity of 

 food within the reach of the parent ; this occur- 

 ring at an early period when the supply is small, 

 and when there may be supposed to be a ne- 



* Mem. pour servir a 1'Hist. des Polypes d'eau 

 douce. Leyden, 1744. 



cessity for the young to move about from place 

 to place in search of sustenance. Sometimes 

 indeed the separation is much retarded, and the 

 young ones also propagate while remaining on 

 the parent stem ; so that the polype assumes a 

 bmnched form, the parent stem bearing families 

 of several generations. 



The Sertularia, Vorticella, Zoantha Ellisii, 

 and Cornularia Cornucopias, also propagate by 

 shoots somewhat in the same manner as the 

 Hydra.* 



Reproduction by separated buds orsporules. 

 The last form of the non-sexual reproduction is 

 that in which the young are formed from small 

 detached masses after they are separated from 

 the body of the parent . These bod i es, general ly 

 of a rounded form, maybe regarded as buds 

 formed in the parent body, as those of polypes 

 are, but detached from it before the evolution 

 of the new animal begins. They bear the same 

 relation to the offspring as the egg of higher 

 animals to their foetus or embryo, and might 

 be regarded as ova but for an important dif- 

 ference of structure to which we shall after- 

 wards advert. They are called sporae, germina 

 granulosa, and gemmae, or germs : they are 

 homogeneous in their structure, and the whole 

 of the substance of which they are composed is 

 converted in the process of their development 

 into the new animal. 



In some animals these sporules are formed 

 in all parts of the body indiscriminately, and 

 are therefore found dispersed through it ; in 

 others there is present a peculiar organ in which 

 they are formed, constituting the simplest form 

 of a reproductive organ. The Actinia, Me- 

 dusae, and some of the lower tribes of Mollusca 

 belong to the first of these sets. In the Aphro- 

 dita the sporules lie in the interstices between 

 the different organs of the animal. In those 

 animals in which a particular organ is provided 

 for the formation of the sporules, the name of 

 ovary is given to that organ, an application of 

 the term not strictly correct, as it belongs more 

 properly to the organ in which complete ova are 

 produced. The production of sporules from a 

 particular generative organ is much the most 

 frequent mode of their formation, and it ob- 

 tains in the greater number of the lower tribes 

 of Mollusca. 



It is a fact worthy of notice that the spo- 

 rules of some Zoophytes, as those of the Sponge 

 observed by Dr. Grant, are endowed with a 

 faculty of moving, sometimes darting with ra- 

 pidity in various directions through the fluids 

 in which they are produced. These motions 

 seem to depend on Cilia: the sporules are also 

 provided with a hook, by which they become 

 attached to other objects when they settle, 



* According to Burdach the propagation of the 

 Volvox globator, already mentioned, and of the 

 Vibrio, Cercaria, and Cysticercus, is effected by the 

 formation of buds, and differs from that of the 

 polype merely in the buds being formed and dis- 

 charged inwardly. We might, perhaps, consider 

 the regeneration of lost parts which takes place in 

 some animals higher in the scale than those pro- 

 pagating by buds as a manifestation in them of a 

 similar power. 



