398 THE CRANIATES AND THE ACRANIATES. 



In every subphylum metamerism is suppressed, the structure simplified, and 

 evolution has of necessity led toward a less active, less complicated mode of life; 

 in fact, farther and farther away from the characteristic condition in the craniate 

 stock, rather than nearer to it. 



The chief characteristics common to the exceedingly heterogeneous group 

 of sub-phyla constituting the acraniates may be defined as follows: 



Metamerism. — The metameres are few in number, except in Amphioxus, 

 and are rarely if ever sharply, or fully defined. Even when the metamerism is 

 fairly well expressed in the younger stages, it degenerates or becomes greatly 

 obscured, or it may disappear altogether in the adult (cirripeds, tunicates). 

 Whether this is a true degeneration, or merely a special form of development, 

 is a matter of definition. It is certain, however, that the possibilities of the meta- 

 meric type of structure, so fully realized in the arachnid division, are never real- 

 ized here. 



There is no organic union of specialized metameres to form a compound 

 head, although in the cirripeds we may recognize one or two pairs of temporary 

 antennae, three pairs of jaws, and five or six pairs of abdominal appendages, 

 indicating the division of the body into tagmas, corresponding approximately 

 to the procephalon, mesocephalon or thorax, and metacephalon or branchial 

 region of the arachnids. (Fig. 275.) In the cirripeds, the primitive head and 

 thorax are the first to lose their metameric structure, the only indication of it 

 left in the adult being the jaws, and even these may disappear. 



The main divisions of the body, but with Httle or no indication of their further 

 subdivision into metameres, are recognizable in the other sub-phyla as the pro- 

 boscis, the collar, and branchial regions (Amphioxus, enteropneusta, ptero- 

 branchia) ; or they are probably represented merely by the principal subdivisions 

 of the coelom seen in the ectoprocta, phoronida, chaetognatha, and echinodermata. 



The appendages are always simple in structure, stub-like, tentacular, or 

 altogether absent. Their serial identity is only vaguely indicated, but we may 

 recognize the rudiments of procephalic appendages, probably corresponding to 

 the antenna; of cirripeds, copepods, and other Crustacea, in the adhesive papilte 

 of the tunicate and echinoderm larvae, and possibly in the tentaculate arms of 

 ectoproctous polyzoa, brachiopods, rhabdopleura, and phoronis. 



The five primordial tentacles of the echinoderms probably represent a group 

 of thoracic, or abdominal appendages (Figs. 291-295); those of Cephalodiscus 

 and the entoproctous polyzoa represent the thoracic, or both circumoral and 

 abdominal groups. (Figs. 299-301.) The appendages may be absent from the 

 outset, or if present they may disappear completely in the adult. After the larval 

 period, they are never used as locomotor organs in any member of the group. 

 In the chastognaths, the cephalic appendages have the appearance of typical 

 arthropod oral appendages. 



