132 COMPAEATIVE ANATOMY. 



way, to the degenerations correlated with, their parasitic habit, 

 and in special points to their relations to their different hosts. 

 Parasitism, in fact, determines the whole pha^nomenou in question, 

 which is spoken of as "alternation of generations," but by no means 

 explained by that phrase. 



§ 103. 



Gemmation is a common process among the Bryozoa also, where 

 it leads to the formation of colonies. As in other Vermes and 

 Coelenterata, the buds are developed from the wall of the body. 

 Accordingly as the bud remains at the side of, and on the same 

 level with, its mother, or grows at one end and raises itself from the 

 ground, flattened or upwardly-growing ramified cormi are foi'med. 

 At the edge of the flattened colony the youngest buds often form the 

 rudiments of several individuals (persons), which are by-and-by 

 separated from one another. We observe in the case of develop- 

 ment by gemmation as also in development from the ovum, that the 

 anterior portion of the body, which carries the crown of tentacles, 

 develops inside the hinder portion of the body, which forms the 

 cell round the animal. The proposal has therefore, though without 

 reason, been made to regard the two portions as separate "indi- 

 viduals." All the persons of a Bryozoan colony are not equally well 

 developed. In many, the parts belonging to the cells and muscles 

 only are developed; these give rise to the so-called Avicularia, 

 which function as prehensile organs for the colony. The Vibra- 

 cularia, which are long spike-shaped structures, continually moving, 

 are parts further modified. Lastly, some persons may serve only 

 for the reception of ova, and form the so-called marsupial cap- 

 sules. Thus we meet here again with a polymorphism, which is due 

 to a division of the physiological work of the colony. 



Appendages. 

 § 104. 



The appendages have the form of actively mobile processes of 

 the body, which may be used for the most varied purposes, accord- 

 ing to their relation to it, and their more special line of development. 

 As low down as the Turbellaria processes are found on the portion 

 of the body which represents the head. In many Planarians lateral 

 lobate processes are developed as tentacles, or feelers; in others 

 the dorsal surface of the body is distinguished by similar processes 

 (Thysanozoon) . 



While the parasitic mode of life of the Trematoda, Cestoda, and 

 many Nemathelminthes causes structures of this kind to disappear 

 from them altogether, such structures are largely developed in the 

 free-living Annulata, and prove to us the great influence of the outer 



