the Insectean Crustacean Types. 17 
pairs of legs to each segment whic 
other related Myriapods*. 
As the true normal limit of the head in an animal is deter- 
mined by the fact that this part includes the senses, mouth, and 
mouth-appendages (for this is demonstrated by the principles of 
cephalization already explained, if not established on other 
grounds), the head in the Decapod includes nine segments, and 
the thorax five, although there is no constriction of the body to 
make the division obvious to the eye. 
The relation of the Insect-type to the Decapod is at once 
apparent from a comparison of the two lines in the preceding 
diagram. Supposing the parallelism rightly presented, the 
following facts are to be noted :— 
1. The Insect-type wants the three posterior segments of the 
Crustacean. 
2. The head and thorax together of the Insect-type have the 
same number of segments (nine) as the head alone of the Decapod. 
3. The head and thorax of the Insect-type contain half of its 
total number of segments (eighteen) ; the same of the Decapod- 
type contains ¢wo-thirds of its total (twenty-one). 
4. The head of an Insect contains six segments, which is one- 
third of the total in the Insect-type; that of a Decapod nine 
segments, or three-sevenths of the total in the Crustacean type. 
[The head of a Tetradecapod, it may be added, contains seven, 
or one-third the total. ] 
5. The visceral segments (or those containing the viscera 
connected with digestion) are the 10th, 11th, 12th, 13th, 14th, 
in both the Insect-type and the Decapod-type. But in the 
Insect the 10th is the first behind the thorax; and in the Crus- 
tacean it is the first behind the head (or the mouth-organs)y. 
occurs in the Judi and some 
* The writer has suspected that the multiplication of segments in the 
Phyllopods might be due to the basal part of each pair of feet becoming a 
separate body-segment, and that the branches corresponded to the double 
feet of the Juli; but, as the members in these multiplicative types appear 
often (if not always) to have the full number of basal joints, this view does 
not appear to be tenable. 
t Only in a degradational group of Decapods (that of the Gastrurans) 
do the viscera reach into the abdominal segments, or those following the 
14th. The abdomen is very much elongated in these species, the cephalo- 
thoracic portion of the body is comparatively small, and the whole strue- 
ture is lax and low in grade. The species thus stand apart from the 
Macrurans as ‘a separate tribe, equivalent to those of Brachyurans and 
Macrurans, while the Schizopods are only degradational Macrurans, See 
‘ Annals,’ /.c. In the fact that the viscera of the Squilloids or Gastrurans 
are contained in the abdominal portion of the animal, this group appears 
to approach the order of Insects. But this seeming approximation comes, 
as observed, through degradation, and is analogous to that between a 
Limulus and an Insect, as explained on page 193. 
Ann. & Mag. N. Hist. Ser.3. Vol, xiii. 2 
