Why konsistanta instead of konsistas?
organo konsistanta el karnaj fibroj kaj servanta por movi la diversajn partojn de la korpo
is ReVo's definition of "muskol/o". (Also reflected here in Simpla Vortaro.)
For dictionary definitions of words or terms, it's not unusual to not be complete sentences and to lack a conjugated verb. This is done in English dictionaries, too, see e.g. wiktionary's entry for "muscle (Noun)":
- (uncountable) A contractile form of tissue which animals use to effect movement.
- (countable) An organ composed of muscle tissue.
- (uncountable, usually in the plural) A well-developed physique, in which the muscles are enlarged from exercise.
- (uncountable, figuratively) Strength, force.
- (uncountable, figuratively) Hired strongmen or bodyguards.
In English, it's implied that you can read the definitions like
<term> can mean / is <definition>
"muscle" can mean a contractile form of tissue which animals use to effect movement.
Similarly, in Esperanto, you can read dictionary definitions like this as
<vorto> povas signifi / estas <difino>
muskolo estas organo konsistanta el karnaj fibroj kaj servanta por movi la diversajn partojn de la korpo
Why konsistanta instead of konsistante?
-e would make it an adverb. An adverb needs an adjective or verb that it applies to. Sometimes, that adverb or verb can be implied instead of explicitly mentioned, but here, I wouldn't even know what it could be.
You can read
organo konsistanta el karnaj fibroj [...]
organo, kiu estas konsistanta el karnaj fibroj [...]
though even without the relative sentence, "konsistanta el karnaj fibroj" and "servanta por movi la diversajn partojn de la korpo" grammatically take the roles of adjectives further describing "organo", thus their main words (konsistanta and servanta) get the adjective ending -a. Remember that normal adjectives in Esperanto can be in front or after the term (usually a noun) they apply to, and having them in front is more common. When the role of an adjective is taken by a phrase consisting of several words, that phrase has to be put after the noun, though.
Actually, this is similar in English. We have:
An organ composed of muscle tissue.
An organ composedly of muscle tissue.
Note that "present continuous" is a term describing a tense of the English grammar and that it requires a conjugated form of the verb "to be".
The form of the verbs-turned-into-adjectives with the -ant- suffix and the -a adjective word ending is called the active participle present. While participles in Esperanto can be used to form tenses analogous to (but much less commonly used than) those in English, they can also be used as adjectives, and that seems to be the case here.