58 



INVERrEBBATJE MORPHOLOQT. 



derivative of the endoderm, and the endoderm of the diploblastic organ- 

 isms is equivalent to the endoderm plus mesoderm of the triploblastie 

 forms. The apparent derivation of the mesoderm from the ectoderm in 

 some of the latter (e.g. Annelida) is to be regarded as resulting from the 

 precocious segregation of the mesoderm at an early period of development 

 and is not to be regarded as indicating its original derivation. 



Non-sexual Reproduction in the Metazoa. — Eeproduction by 

 division and by budding, though playing by no means so im- 

 portant a part as in the Protozoa, is neverthe- 

 less of frequent occurrence in the Metazoa, 

 especially in certain groups. In certain Tiir- 

 bellarian worms (Microdoma) division is the 

 usual mode of reproduction, replacing almost 

 completely the sexual method, and, the indi- 

 viduals so produced remaining in connection 

 ■with one another, longitudinal chains are 

 produced, consisting of individuals in various 

 degrees of separation (Fig. 28). In certain 

 Annelids also (Naididse) division frequently 

 takes place, occasionally each metamere being 

 capable of developing into a new animal, as in 

 Ctenodrilus. 



Budding, however, is a rather more fre- 

 quent method and is characteristic of certain 

 groups, such as the Hydroids, Anthozoa, and 

 Fig. 28. — D I a- Bryozoa. In some cases, as in Hydra and 

 GKAM SHOWING ^^^^ medusis, the buds separate from the 



NON-SKXDAL Rb- tit ■ t t 



PRODUCTION OF P^^rent and lead an independent existence ; 

 A ToRBBLLAKiAN but frequently the separation is not complete, 

 WoKM Microsio- resulting in the formation of colonies the 

 individual components of which are in or- 

 ganic connection with each other. In such 

 colonies a physiological division of labor among the con- 

 stituent individuals may take place, as in the Hydroid 

 Ifydractinia (see p. 87) where some of the individuals devote 

 themselves to the nutrition of the colony, others to its 

 reproduction, and others again to the protection of their 

 weaker companions. The assemblages produced by bud- 

 ding may assume very complicated shapes, though occa- 



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