566 DEVELOPMENT OF ECHINODERM AT A AND ENTOZOA. 



anal orifice, their larval zooids, like those of other Echino- 

 derms, always possess both. It is from the side of the intes- 

 tinal canal, that the young Echinoderm is usually budded-off. 

 In some instances it separates itself completely from the 

 zooid, when it has attained a certain stage of development, 

 no part of the latter entering into its composition ; this seems 

 to be the case in the Comatula, which, present this further 

 remarkable feature, that the young Echinoderm at first 

 attaches itself to some fixed object by a footstalk (fig. 305), 

 so as to resemble the fossil Encrinites in every essential 

 particular, but afterwards becomes detached, and henceforth 

 remains free (fig. 306). In the Starfish and Echinus, the only 

 part of the larval zooid which is retained in the Echinoderm, 

 is a portion that is (as it were) pinched-off from the stomach 

 and intestines. In the Holothuria (fig. 67), on the other hand, 

 which has a much closer conformity to the type of the Annelids, 

 a much larger part of the larva is retained in the adult, and 

 the process of development more nearly resembles an ordinary 

 metamorphosis. 



742. Passing-on now to the Articulated series, we find 

 that the developmental history of its lower forms presents 

 phenomena not less remarkable than those already noticed, 

 feecent studies on the propagation of the Entozoa have re- 

 moved many of the difficulties previously felt -in regard to 

 their mode of passage from one animal to another ; by 

 showing that the same creature may exist under two or more 

 forms, which may differ so greatly from each other as appa- 

 rently to belong to separate orders. This has now been 

 ascertained to be the case, for example, in regard to the 

 Tcenia or Tape- worm (fig. 53) and the Cysticercus (fig. 307). 

 The segments of which the body of the Tape-worm is composed, 

 are in reality repetitions (like the medusa-buds of a Hydroid 

 zoophyte) of its generative apparatus ; with this difference, 

 however, that each segment contains both kinds of sexual 

 organs, so that the eggs it contains are fertilized without any 

 extraneous assistance. These segments, when mature, detach 

 themselves one by one ; and being voided from the intestine, 

 fall to the ground, over which the eggs they contain become 

 disseminated by various agencies. Being swallowed with 

 the herbage or the water ingested by herbivorous animals, 

 the eggs are conveyed into their stomachs, where* the little 



