THE GENUS PTERYGOTUS 197 
This number cannot by any means be made out in all Cofe- 
poda, but I know of no members of the order which exceed it. 
Different opinions have been entertained with respect to the group- 
ing of these somites into cephalic, thoracic, and abdominal. The 
eyes, antennule, antenne, and the two anterior pairs of post-oral 
limbs, and with them their somites, are indeed universally ad- 
mitted to belong to the head; but the two following appendages 
are considered by some to be maxillz, by others to be maxilla and 
maxillipedes ; by yet others to be manillipedes and anterior thoracic 
limbs. 
The simplest and most natural view appears to me to be, to 
regard the third pair of post-oral appendages as the homologues of 
the second maxilla or last cephalic appendages, while the fourth 
post-oral appendages are thoracic members. -In this case there will 
be six pairs of thoracic appendages, as in the Crrripedza. If we 
inquire what has become of the two thoracic somites which are unre- 
presented by appendages, three probabilities present themselves :— 
1. They have remained undeveloped at the anterior end of the 
thorax; or, 2, they have remained undeveloped at the posterior 
end of. the thorax; or, 3, the two somites which are so closely 
united together as to appear as one segment, commonly regarded 
as the first of the abdomen, belong in reality to the thorax. This 
view might be supported by the position of the reproductive 
apertures which open behind the second of these somites, that is, 
on this hypothesis, in that position immediately behind or at the 
posterior part of the thorax, which is so common to them in 
Edriophthalmia, Pecilopoda, and male Podophthalmia. If this view 
be correct, (but I would expressly state that it is put forth only tenta- 
tively,) it is the abdomen alone which is shorn of its due proportions 
in Copepoda. 
Such is the structure of a typical copepod. The modifications 
observable in the order, of importance for my present purpose, 
affect,—1stly, the form of the body; 2ndly, the character of the 
eyes; 3rdly, the form of the antennules and antenne ; 4thly, that 
of the labrum and metastoma; 5thly, the number of the thoracic 
appendages. 
1. The body usually has a general resemblance in outline to that 
of Calanus, the abdominal somites commonly presenting a marked 
and immediate diminution in transverse diameter when compared with 
those of the cephalo-thorax. In the beautiful Sapphirine (Plate XVI. 
[Plate 27] fig. 19, however, the passage from the cephalothoracic 
to the abdominal somites is quite slow and gradual. 
