the Insectean and Crustacean Types. 17 



pairs of legs to each segment whicli occurs in the luli and some 

 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 theDecapod. 



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 two-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) f. 



* 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 lull ; 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 struc- 

 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,' I. 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. Fo/.xiii. 2 



