后缀-acea ，来源于拉丁语，以这个为后缀的单词，一般都指一大类目，而不是指个体，例如：cetacea 鲸类、testacea 介壳类、crustacea 甲壳类。以-acean为后缀的单词，既指动物族群或纲目中的个体，也形容与它们有关的。最常见的例子例如cetacean 鲸类的，crustacean 甲壳类的。

英语词源：
[The neuter plural of the Latin ending -aceus, belonging to, or of the nature of.]

Terms in -acea are systematic names of animal groups, mostly orders but also classes and a few superfamilies. Examples include Crustacea, a class of mostly marine arthropods that includes lobsters, shrimps, crabs, and barnacles; Cetacea, the order that includes the whales, dolphins, and porpoises; and Cumacea (Greek kuma, billow or wave), the order containing the tadpole shrimps.

Names in -acean either indicate individual members of these groups or adjectives relating to them. The most common examples are cetacean and crustacean; another is larvacean for a member of the Larvacea, a class of minute transparent planktonic animals related to the sea squirts. See also -ae and -ean.
---------------来源 Michael Quinion’s book Ologies and Isms.

-acea,suffix denoting orders and classes in zoology.—L.-ācea,neut. pl. of-āceus,'belonging to,of the nature of;see -aceous .The neut. pl.form of suff. -āceus is used because it refers to L.animālia ('animals'),pl.of the neuter noun animal. Cp. -aceae .
---------------克来茵英语词源字典