We shall, therefore, ignoring the ocular somite, speak of the first, second, third, fourth, fifth and sixth legbearing somites of the prosoma, and indicate the appendages by the Roman numerals, I, II, III, IV, V, VI, and whilst ignoring the praegenital somite we shall speak of the first, second, third, &c., somite of the mesosoma or opisthosoma (united mesosoma and metasoma) and indicate them by the Arabic numerals.
Intermediate somites forming a mesosoma occur, but tend to fuse superficially with the metasomatic carapace or to become co-ordinated with the somites of the metasoma, whether fused or distinct to form one region, the opisthosoma (abdomen of authors).
- One of the Nymphonomorphous Pantopoda, Nymphon hispidum, showing the seven pairs of appendages I to 7; ab, the rudimentary opisthosoma; s, the mouth-bearing proboscis.
Not freeswimming, none of the prosomatic appendages modified to act as paddles; segments of the mesosoma and metasoma (= opisthosoma) not more than ten in number, distinct or coalesced.
- Free-swimming forms, with the appendages of the 6th or 5th and 6th pairs flattened or lengthened to act as oars; segments of mesosoma and metasoma (= opisthosoma), twelve in number.