438 HISTORICAL GEOLOGY. 



decephalization and a multiplicate or nearly limitless segmentation are extreme. There is 

 no lower extreme, except in that of the compound mass of the sponge or the polyp, where 

 a head fails entirely. 



In the higher typical Worms, the Annelids, the many segments of the body have a 

 separate nervous ganglion as an enlargement of the nervous cord, or pair of cords, that 

 passes posteriorly from the cephalic ganglion, giving it a degi-ee of independence. But the 

 head has its mouth, and the body its intestine and reproductive system, so that the 

 structure is one in system of growth and reproduction. Yet the number of body-segments 

 varies greatly, it being not often fixed even for the species of a genus ; and all of the 

 segments behind the head participate alike essentially in the work of locomotion. The 

 body structure in Worms is, therefore, multiplicate, and greatly decentralized, and loco- 

 motion is of the diffuse kind. Moreover, jointed limbs are wanting. 



2. Crustaceans. — Crustaceans contrast strongly with the Worms, high or low. (1) The 

 body consists of a head (which, as in other animals, includes the mouth as well as the 

 organs of the senses), a thorax furnished with limbs, and an abdomen. (2) The number 

 of body-segments, in typical species, is limited, instead of being multiplicate, it not 

 exceeding 20 ; 6 of these segments pertain to the abdomen, and 14 to the thorax and 

 head. 



On differences in the arrangement and functions of the parts of the structure, 

 exemplifying degrees in cephalization, is based the accepted system of classification in 

 Crustaceans. This system subdivides the typical species into (1) Decapods, or the 

 10-footed, and (2) Tetradecapods, or the 14-footed ; and each of these tribes into two 

 subordinate groups, Brachyurans and Macrurans for the former, and Isopods and 

 Amphipods for the latter. 



Decapods and Tetradecapods. — In the Decapods (1) the head includes 9 body- 

 segments — the 3 anterior bearing the organs of the senses, the eyes, and 2 pairs of 

 antennae, and the remaining 6, the jointed mouth organs ; and (2) the thorax, comprising 

 the remaining 5 segments of the cephalothorax, bearing 5 pairs of feet. In the Tet- 

 radecapods, the head corresponds to only 7 body-segments, and has, therefore, but 4 pairs 

 of mouth organs, while the thorax includes 7 segments and bears 7 pairs of feet. In other 

 words, the anterior 2 pairs of feet of the Tetradecapods are pairs of month organs in the 

 Decapods. There is thus a transfer forward of legs to the mouth series on rising from 

 the Tetradecapod tribe to the Decapod. It is a case of concentration headward in the 

 structure, or of higher cephalization. The two tribes also differ in the mean size of 

 the animals. Decapods having, on an average, 100 times the bulk of Tetradecapods, 



We pass now to the two subdivisions of the Decapods and Tetradecapods. 



Brachyuran and 3Iacrnran Decapods. — A Macruran Decapod, as exemplified in a 

 Lobster, Prawn, or Shrimp, has (1) an elongate, loosely compacted body, with the 

 abdomen nearly as long as the cephalothorax, and in some species several times longer ; 

 (2) the abdomen is the most powerful organ in locomotion ; (3) the thoracic feet are 

 feeble in locomotion ; (4) the outer mouth organs are foot-like, free, and long ; (5) the 

 antennae are sometimes a foot or more long. 



The Brachyuran, as the common Crab, has, on the contrary, (1) a short body, it 

 being seldom longer than broad ; (2) the abdomen in males is' very small and narrow, 

 it doing no service in locomotion, but, instead, lying confined in a groove on the under 

 side of the body, so that the animal is almost comprised within the first 14 of the normal 

 20 segments of the Crustacean ; (3) the thoracic feet, or those of the posterior 5 of 

 these 14 segments, are the sole organs of locomotion ; besides (4) the mouth organs are 

 small, and closely stowed away together within, or over, a shallow cavity, which is 

 covered by the outer pair, as an operculum ; and (5) in harmony with the general com- 

 pactness of structure, the antennae are very small, seldom exceeding an inch in length. 



