Tagma

This group of four neuromeres constituting the procephalic lobes is further marked off from the rest of the plate by its height, and also by its darker color as in previous stages. These distinctive characters make it convenient to apply the term "tagma" suggested by Lankester for a more or less isolated and independent group of segments. The aptness of this term will become more apparent as the development of the procephalic lobes is followed in the succeeding stages and when it is discussed in the conclusion.

Source: wiktionary

The existing Arachnida, like the higher Crustacea, are "nomomeristic", that is to say, have a fixed typical number of somites to the body. Further, they are like the higher Crustacea, "somatotagmic," that is to say, they have this limited set of somites grouped in three (or more) "tagmata" or regions of a fixed number of similarly modified somites — each tagma differing in the modification of its fixed number of somites from that characterizing a neighbouring tagma. The most primitive among the lower Crustacea, on the other hand, for example, the Phyllopoda, have not a fixed number of somites, some genera — even allied species — have more, some less, within wide limits; they are "anomomeristic". They also, as is generally the case with anomomeristic animals, do not exhibit any conformity to a fixed plan of tagmatism or division of the somites of the body into regions sharply marked off from one another; the head or prosomatic tagma is followed by a trunk consisting of somites which either graduate in character as we pass along the series or exhibit a large variety in different genera, families and orders, of grouping of the somites. They are anomotagmic, as well as anomomeristic.

Source: wiktionary

1889 Prof. J. S. Burdon Sanderson: Elementary Problems in Physiology. In: Annual Report of the Board of Regents of the Smithsonian Institution, 1890. His view is that certain well-known properties of organized bodies require for their explanation the admission that the simplest visible structure is itself made up of an arrangement of units of a far inferior order of minuteness. It is these hypothetical units that Nägeli has called Micellae. Now, Nägeli, in the first instance, confounded the micella with molecules, conceiving that the molecule of living matter must be of enormous size. But inasmuch as we have no reason for believing that any form of living material is chemically homogeneous, it was soon recognized, perhaps first by Pfeffer, but eventually also by Nägeli himself, that a micella, the ultimate element of living material, is not equivalent to a molecule, however big or complex, but must rather be an arrangement or phalanx of molecules of different kinds. Hence the word Tagma, first used by Pfeffer, has come to be accepted as best expressing the notion. And here it must be noted that each of the physiologists to whom reference has been made, regards the micella, not as a mere aggregate of separate particles, but as connected together so as to form a system...

Source: wiktionary